The 5MM/35 SMc™ Cartridge
in a Custom Savage Model 12
By Norman E. Johnson
McPherson comments on this article in this font style and color:
No load listed anywhere within this article is represented as being safe for use in any other rifle. Always work up loads carefully, using prudent handloading practices.
Recently, our acquaintance, Norm Johnson, submitted the following piece to Varmint Hunter Magazine. We have obtained permission to offer this article on our web page.
Evidently, we failed to let Mr. Johnson know that all the loading data that we provided was based upon the use of molybdenum-disulfide plated bullets. My significant 5/35 SMc testing shows that moly-versus-naked bullets typically account for about 1½ grains difference in charge mass, to achieve similar peak chamber pressure; moreover, plated bullets typically reach at least 25 fps greater velocity, when loaded to any given peak pressure.
I have taken the liberty of doing a bit of editing, so that this work will reasonably well match the style of other pieces on this site. For example, the leading decimal in caliber designations — e.g., 20-caliber — is erroneous and I have deleted all such decimals. Similarly, the correct designation is 223 Remington, not .223 Remington. I have also clarified one section of Johnson’s text, in an effort to represent more clearly the technical points involved.
Within this article, where appropriate, I have added comments using this font (red, italic).
Perhaps few rifleman or hunters really think much about what goes into the design and development of the cartridges they use. They may not realize that any cartridge worthy of a headstamp must undergo a lengthy period of planning and research; or that the search for a better cartridge may be wrought with considerable setbacks preceding its eventual announcement and acceptance by the shooting public.
This might suggest the question of why the sudden spark of enthusiasm for such cartridges as we commonly see in any caliber. This is particularly true following decades with no apparent need or interest for such introductions — especially in a new or revised caliber. Yet, cartridges come and go; and sometimes they fade away, only to return.
In Parker Ackley’s Handbook for Shooters and Reloaders, dated 1962, the 20-222 is shown. Custom-made bullets of 45- and 48-grain weights are listed at 3,450 and 3,335 fps, respectively. Then, around 1963, gun writer Bob Forker originated the 5mm-223. It was made by necking the 223 Remington down to accept 0.196-inch diameter bullets that he formed with special bullet dies. Velocity around 3,550 fps was shown with 30-grain bullets and near 3,500 fps with 33-grain bullets. Surely, such an exciting cartridge should have caught the eye of some eager manufacturer just waiting for such a new announcement. I doubt if many of the 20 caliber forerunners would have predicted back then the surge in variant sub-caliber cartridges as we are seeing today.
More recently, there have appeared several other 20-caliber entries. As with most wildcat versions, such cartridges were based on existing cases formed and necked down to bullet diameter of choice. Notable among these current sub-calibers are the 20 BR, 20 PPC, 20 VarTarg, 20 Tactical, and, of course, the commercialized 204 Ruger, developed in 2003. Though the cartridge name pretty much identifies its origin here, I will briefly describe the parent case headstamp, which spawned these five sub-caliber cartridges.
The 20 BR is based on the 6mm Norma BR case, which is readily necked to 20-caliber. This case will move a 40-grain bullet at around 4,000 fps. The 20 PPC, of course, is based on the famed 6mm PPC benchrest cartridge, which originated from the 220 Russian case. The 20 PPC will move a 40-grain bullet at more than 3,900 fps and a 30- to 32-grain bullet at 4,200 fps, depending on bullet type and rifling twist as well as other factors. The 20 Tactical cartridge is built on the 223 Remington case, which appeared an open invitation to squeeze this case neck down to 20 caliber. Brass is available by the basketful and sizing this relatively short neck is quite easy. The 20 Tactical has no problem moving a 30- to 32-grain bullet at 4,000 fps or a 40-grain bullet in the 3,800 to 3,900 fps velocity range.
Included among the somewhat lesser case capacity versions of the 20-caliber is the neat little 221 Fireball, necked down. This one is dubbed the 20 VarTarg, which stands for Varmint and Target. I have had a number of the Remington XP-100 handguns chambered for the 221 Fireball, as well as other smaller cartridges, and they really shoot. Currently I have an XP-100 as a switch barrel setup of my own doing in calibers 221 Fireball, 222 Remington, and 223 Remington. Both of the 0.224-inch bores will shoot very tight groups under half minute of angle with the 16-inch varmint weight barrels. At a later date, I can see one of the 20-caliber cartridges joining this trio on the very capable XP-100 action. As a single-shot varmint rifle, this cartridge and action would have few rivals. The 221 Fireball case, necked to 20 caliber, is very efficient and still capable of moving 30- to 32-grain bullets in excess of 3,700 fps. Recoil and noise are markedly reduced with the 20 VarTarg.
Outside the parameters of wildcat cases, in 2003, Hornady and Ruger teamed up to introduce the 204 Ruger as a commercial round. This cartridge is designed to accept 0.204-inch diameter bullets. Ammunition and bullet manufacturers lost no time in offering loaded ammo, cases, and bullets for the 204 Ruger. Among the riflemen that I know personally, I am the only one clinging to a rifle chambered for the 222 Remington Magnum cartridge. I still have a Remington Model 40-X in 222 Rem. Mag. as a switch-barrel rifle and it is a shooter. I was not surprised to learn this cartridge was chosen by Messrs Hornady and Ruger as the parent case for the 204 Ruger.
ANOTHER NEW 20-CALIBER CARTRIDGE — THE 5MM/35 SMc
This cartridge entered the scene as one of a series of patented and copyrighted cartridges including 17, 20, 22, 24, 26, and 30-caliber versions introduced by Messrs Byrom (By) Smalley and M.L. (Mic) McPherson of Superior Ballistics Inc., 230 Skyline Drive, Brigham City, Utah 84302. This series included a provocative looking cartridge in 20 caliber, which was head-stamped 5mm/35 SMc and is based upon the 6mm Norma BR case. Savage Arms had built four custom Model 12 rifles and four of us gun writers would later be involved in the testing and evaluation of the new round. Considerable preliminary research and testing had already been performed by Smalley and McPherson, involving a number of different barrels and rifles, along with the ballistics involving cartridge design, before the new cartridge would receive early, independent testing in the hands of others.
It was in the summer of 2005 when John Anderson, editor of The Varmint Hunter Magazine, and By Smalley and M. L. McPherson communicated with me regarding the testing and evaluation of this cartridge.
Here I shall describe in detail the premise of the new 5mm/35 SMc cartridge as well as how it applies to this series of patented and copyrighted cartridges. So here is my interpretation of some of the written text, as given to me.
The Smalley-McPherson cartridge design follows the lines of the short-fat cartridge theory with an ellipsoidal (radiused) shoulder similar to that of a Weatherby cartridge. It differs in this respect in that the Weatherby case has both inner and outer radii, one at the neck-to-shoulder junction and one at the shoulder-to-body junction. The new 5mm/35 SMc cartridge does not have a (significant) radius at the neck to shoulder junction. These descriptions would be an oversimplification, so I will cover in much detail this cartridge design, and how the inventors describe its internal combustion.
The very large inner (shoulder-to-neck) radius used on all classic Weatherby chamberings is the worst possible shoulder design because this feature works against trapping unburned propellant within the case; rather, — compared to a conventional conical shoulder — the Weatherby design facilitates the flow of more unignited propellant into the bore, thereby reducing efficiency, increasing barrel heating, and increasing the initial rate of gun acceleration, which results in greater felt recoil. Among exiting commercial chamberings, this is, quite literally, the worst possible design for a shoulder. MLM
The short-fat cartridge theory places great emphasis on the ratio between bullet diameter and overall case diameter. The case-shoulder configuration is equally important to internal ballistics in the SMc line of cartridges. So, what really happens inside a cartridge?
As a cartridge is analyzed during ignition and burning of the powder, the following is reported to take place. Hot particulate substance and gas originating from the primer mixture pass through the primer flash hole directly into the powder chamber at very high velocity. These flaming hot particles and gases serve as the primary powder ignition source immediately forward of the flash hole, but the intensity of the primer heat and gas is cooled by the process. In conventional cartridges, the initial primer flash usually does not contain sufficient heat to ignite the entire charge of powder within the case. Those grains within the powder mass, not initially ignited with air trapped between the granules, are compressed into a mass that is further driven to the front of the case, behind the bullet. This mass of compacted air-entrained powder is so firmly compressed that only poor ignition is the likely result. In addition, the compressed propellant pushes on the bullet base, possibly starting it in motion, depending on case volume, bullet neck tension, etc. If the primer blast force does not move the bullet, the pressure rise in the fixed burning volume of the case soon does.
The brass case wall has high heat capacity and thermal conductivity much greater than that of smokeless powder. The high heat transfer occurring at the propellant-to-wall-to-fire junction has the effect of retarding burning of propellant in contact with the case wall. Further, when the bullet begins to move, a shear line approximately the diameter of the bullet is set up in the mass of propellant that is pushing on it. Progressive burning then takes place rapidly along the developing shear surface, as that breaks through the already ignited rearward surface of the compressed propellant mass. Burning thereafter progresses radially toward the sidewall and center of the case, as well as forward in the case at the base of the bullet.
As burning intensity increases, a portion of the propellant plug or mass is being thrust into the rifle bore along with the bullet. In straight-wall cases, propellant that is not ignited by the primer can burn only from the rear end forward because of the retarding effect at the case wall and because secondary heating cannot ignite granules within that mass. Straight-wall cases therefore require significantly faster burning powders to attain full pressure.
Further claims for the patented SMc line of cartridges is reduced barrel heating, as reported in the October 2005 issue of The Varmint Hunter Magazine. A lengthy study by M. L. McPherson concluded that efficiency of any chambering is a critical factor in barrel heating. Many charts and graphs were shown to help illustrate this, which may or may not be understood by anyone but a science-minded person. In my own experience during the testing of the 5mm/35 SMc cartridge, there appeared to be a similar amount of barrel heat present as with other similar case-to-bore volume cartridges. This may be a moot point, however.
Moot Point: That which is subject to discussion, disagreement, or argument but that will not be considered here. Okay, I will not argue the point here, I will simply reiterate the most pertinent results of my, carefully conducted, scientific testing: When tested under controlled, identical conditions, in identical barrels, factory 204 Ruger loads and handloads for the 5mm/35 SMc generated essentially identical barrel heating, despite the fact that the 5mm/35 SMc loads produced more than 6.0% greater muzzle velocity (tested loads with both 32- and 40-grain bullets generated fully 13% greater muzzle energy — in the 32-grain loadings, the 5/35 generated fully 10% less barrel heating per shot, in the 40-grain loadings, the 5/35 generated no more than 3% greater barrel heating per shot. Based upon this testing, there can be no argument, the SMc design definitely generated significantly less barrel heating. MLM
Case Shoulder Angle: A Significant Factor
The shoulder angle of a cartridge influences the focus or direction of the shock waves initiated by the primer. Low shoulder angles tend to direct the shock waves toward the base of the bullet. Not only does this contribute to bullet movement before pressure buildup, but it transfers heat energy to the base of the bullet, which would better be utilized in preparing propellant for ignition. In shorter cartridges with steeper shoulder angles, more of the shock is reflected back into the propellant volume, immediately behind the base of the bullet, compressing and heating that material.
This all adds to the improved efficiency of SMc and similar cartridge designs. MLM
Conversely, in a long, slender case, propellant trapped behind the case shoulder burns toward the sidewall; this burning occurs in a region of maximum pressure and is, therefore, relatively rapid. However, burning of the propellant layer that is in contact with the case wall is significantly retarded, due to rapid heat loss to the case. Because the mass behind the bullet is accelerating down the bore, as it pushes the bullet, it is subjected to significantly less pressure (lower temperature) and, therefore, it burns slower.
As noted, heat losses to the case sidewall, neck, and barrel tend to slow overall combustion of contacting propellant. Conversely, in a short-fat case of the same volume, the propellant cylinder trapped behind the shoulder is thicker and, therefore, continues to burn longer, with less retardation (less propellant is touching the case); similarly, the propellant plug moving down the bore contains less propellant. Thus, more gas is generated sooner and less energy is consumed accelerating unburned propellant down the bore.
As described in the text, pictures graphically show how energy from the primer blast shock wave is affected by cartridge shape and design.
The SMc cartridge designs by Smalley and McPherson have unique, ellipsoidal shoulders (SMc shoulders are designed to focus the primer shock waves into the propellant volume, just behind the bullet). This maximizes heating of entrained air in the propellant that is last to ignite and burn, as that mass pushes the bullet down the bore.
As stated by the inventors of this cartridge design, it does not matter if the bullet base is set out in the neck after seating, but no part of the bullet should be seated significantly below the neck-shoulder junction, else it will interfere with shock waves and prevent maximum propellant heating. SMc cases are identified by nominal bore diameter and case volume, in grains of water, to the inside junction of the shoulder and neck, where the bullet base should, ideally, be seated. Thus, we have the 0.204-inch (5mm) bullet with 35 grains of usable water capacity as the 5mm/35 SMc.
So, with this introduction to the various 20-caliber cartridges and a bit more insight on cartridge design and development, and resultant internal ignition and burning characteristics, it is time to see how this new case design fared — at least in the hands of one rifleman.
After the necessary communication and preparation for receiving the needed items for testing the new 20-caliber cartridge, I received a custom Model 12 Savage single-shot rifle in early August of 2005 from the Savage factory. Soon after, three brands of 20-caliber bullets arrived consisting of those then available offered by Hornady, Berger, and Sierra. The Hornady bullets were 32- and 40-grain V-Max. In Berger, I went with their 30-, 35-, and 40-grain hollow-point varmint bullets. Sierra offers 32- and 39-grain hollow-point bullets in their popular BlitzKing series. Too late to be included in this evaluation, I later received some 40-grain Ballistic Tip bullets from Nosler. All these bullets had the appearance of being high-class performers.
Through Mr. Smalley, I received a set of Wilson reloading dies and a small supply of formed 5mm/35 SMc cases bearing the 6mm Norma BR headstamp — the parent case.
In preparation for testing, I went over the Savage rifle to be sure it was not lacking any of the pre-testing requirements I expect of any rifle. This included a good inspection of the bedding and to be sure the free-floated barrel had ample metal-to-wood clearance fore and aft. The trigger (an AccuTrigger) was set to minimum pull, which was nice and crisp. I was already familiar with the unique AccuTrigger and it is a wonderful innovation for any serious rifleman — I learned that McPherson had input in the design of this trigger, he was solely responsible for the incorporation of a secondary safety interlock that positively prevents firing the gun after the sear fails, without literally breaking something by deliberate abuse of the mechanism.
Among my test scopes I usually have a few available that are unmounted so I elected to go with a straight 36 power Leupold BR scope that would sit nicely in the 1-inch Weaver-style rings to fit the scope bases provided with the rifle. The scope has a fine target reticle, and parallax settings for all ranges to 25 yards. The scope barely cleared the husky target barrel, which was just fine.
Few rifles get by my preliminary testing routine without a good inspection of the inside of the barrel and more recently, my pre-testing barrel break-in routine. The Hawkeye bore scope revealed a very nice looking bore with minimal machining marks. There were present some reamer marks in the leade (throat) of the bore, but the leade had a very uniform rifling land configuration so I elected to leave things unchanged.
Before my receiving the rifle, Savage had done some proof testing of the gun. Two fired cases accompanied the rifle, along with a test target showing a sub half-inch group. Load data did not accompany this target, nor distance at which the test target was fired. The ejector was missing on this rifle bolt, which made little difference in its function as a single-shot. I felt this part was intentionally left out so did not question it.
Savage does all accuracy testing at 100-yards. I am rather surprised the factory shipped a bolt, sans ejector. It was not deliberate. I wonder how this oversight happened. MLM
Having tested many scores — even hundreds — of rifles, I have developed a sort of gut feeling that if a rifle or load was going to shoot, this would be recognized quite early in its testing. Further, feeding round after round through a rifle that was not responding was like beating a dead horse. Over the years there had been clear evidence lending probability to this belief.
The Savage Model 12 is a very heavy single-shot, with a heavy barrel and a 1:12 rifling pitch. The lands and grooves are of similar width, and show good square corners and hardly any tooling marks that often are common among many production barrels.
Close examination of the Norma cases revealed turned case necks, which were very uniform at around 0.015-inch. The pair of fired cases received from Savage with the rifle showed uniform case necks at 0.014-inch. Case length overall ranged from 1.552-inch to 1.561-inch. I would shoot these as received, with no plans to trim for length uniformity. The case headstamp was 6mm Norma BR, and later I received some Lapua cases as some of the Norma brass developed enlarged primer pockets because of soft case heads.
Before testing the Savage rifle, I had received a limited listing of loads tried in different rifles with different barrels and rifling twists from Smalley and McPherson. These loads showed an extensive variety of powders and rifling twists, using the same bullets I had at my disposal, namely the Hornady, Berger, and Sierra.
It was not in my plan to try the many loads and variables, which were shown me by Smalley and McPherson, but I did heed some of the warnings and suggestions they were sending me via e-mail, as guidelines in my testing. Some pretty potent loads were included among them, with velocities, and I would treat these with utmost caution. My primary objective was to try to determine a few good varmint loads that were both accurate and at velocities that were realistically safe and sensible. I believe this was accomplished with the rifle and components I had. At this point, both Smalley and McPherson were vacillating among what might be the best powders, bullets, and rifling twist, as the cartridge was still in its formative stages of load work. These included barrel twists ranging from 1:16, 1:14, 1:13, 1:12.5, and 1:12. Bullets at ultra-high velocity at certain twist ratios were reportedly going to pieces in flight because of rotationally induced stress. So, it is understandable if you do not see some of these velocities in my testing — even though I did approach some with caution. I had communicated with one well-known writer who was using a 20 BR cartridge and 50-grain bullets that required a minimum 1:10 rifling twist. A 1:12 twist, of course, would not stabilize a 50-grain bullet, at least not the longer ones. Included in Table I are a number of loads used by the inventors of this cartridge.
Subsequently, we have concluded that the various 39- and 40-grain bullets now offered require a 1:12 twist. The new 40-grain Nosler BT shows great promise and the forthcoming 32-grain version of the BT will likely work perfectly at maximum velocity in the 1:12 twist. Excepting the BT, all bullets lighter than 40 grains will usually fail when launched at maximum feasible 3/35 SMc velocity. MLM
With the bullets I had, Hodgdon’s Benchmark powder was performing very well. Velocities were uniform and accuracy appeared to be good, within a prudent and sensible velocity range. I would later experiment with ball type powder for comparison, namely TAC, offered by Western Powders.
The first groups tested were shot at 50 yards as I was sighting-in the rifle. I used a 39-grain Sierra BlitzKing bullet and 30 grains of Benchmark powder. Velocity was around 3,900 and two three-shot groups were at an incredible one-quarter inch.
I then cleaned the rifle with Shooter’s Choice and the bore looked good, with no copper fouling to speak of at this early stage. With bullet seating between 0.010-inch and 0.015-inch off the lands, I gradually increased the powder charge using different bullets and began shooting at 100 yards. I repeated the 30-grain Benchmark powder charge with the 39-grain Sierra BlitzKing bullet and got a velocity average of 3,881 for five shots, and a very good 0.362-inch group. Further increase in charge to 31 grains of Benchmark increased velocity average to 4,007 fps and a 0.278-inch group. Pressures appeared acceptable with minimal primer cratering around the firing pin.
I then switched to the 32-grain Sierra BlitzKing bullet with the same bullet seating length (around 0.015-inch) off the lands. Here, 32 grains of Benchmark powder filled the case to the base of the neck. Velocity averaged 4,362 fps with a group of just over half an inch. Increasing the charge to 33 grains began to open up the group and increase velocity to around 4,410 fps. Following this load, I went to one of the maximum charges of Benchmark powder given me by Smalley for 32-grain bullets. This was 34 grains of Benchmark, and I was still using the 32-grain Sierra BlitzKing bullet. This load produced a slightly compressed powder charge. I guess I had to know how this rifle would react as loads approached the velocity level Smalley and McPherson were getting. I loaded five cases with 34 grains of Benchmark and shot a 100-yard five-shot group.
As expected, accuracy opened up to 1.067-inch and high pressure signs immediately appeared. Velocity increased to between 4,557 and 4,617 fps. Heavy primer cratering was noted, and primer pockets opened up enough for some of the primers to fall out. Bolt lift was noticeably difficult — all the signs that told me to back off. Surely, these case heads were soft.
Evidently, Mr. Johnson was not using moly-plated bullets, as we had for all our testing. My 5/35 SMC testing shows that, when compared to otherwise identical loads using moly-plated bullets, naked bullets require a charge reduction of about 1½ grains (to generate the same peak chamber pressure); similarly, such naked-bullet loads will typically generate at least 25-fps less velocity.
The heads on cases from this production lot are, indeed, unusually soft. Loads that I had been using in older cases that generated zero case-head expansion and no untoward pressure signs resulted in ruined cases when used in cases from this lot. MLM
On inspection of the bore, there was noted a fairly heavy copper buildup from the muzzle rearward for about 8 to 10 inches. A 10-minute soaking with Barnes CR-10 Bore Cleaner quickly removed the copper, as it always does when I use this method.
Switching bullets from the Sierra to Hornady V-Max and Berger showed the same trend relative to velocity increase versus accuracy. Not unlike any other bore, the 20-caliber tends to enlarge groups as velocity increases. Higher velocity also accentuates bore fouling in any bore.
This is only necessarily true when something is failing. In this instance, the bullets were failing, due to excessive rotational speed. MLM
I later received some formed and outside-neck-turned Lapua cases head-stamped 6mm Norma BR. Here I shall describe my findings using the 40-grain Berger boat-tail bullet, the 39-grain Sierra BlitzKing, and the Hornady 40-grain V-Max bullet. I wanted to see how near-maximum loads with these heavier bullets would perform with regard to velocity, pressure signs, and accuracy in my assessment of the capabilities of this cartridge and rifle.
I began with the Berger 40-grain boat-tail bullet with plans to work up to the maximum load given me by Smalley for Benchmark powder. Ambient temperature was around 32 degrees F during testing, and cloudy with very light wind. The first load I used was 31.5 grains of Benchmark, which produced velocities of 4,029, 4,066, 4,037, 4,048, and 4,027 fps. Bolt lift was easy with just a hint of primer cratering over the firing pin, and accuracy of 0.780-inch at 100 yards. The charge was increased to 32 grains of Benchmark and produced velocities of 4,165, 4,155, 4,137, 4,160, and 4,190 fps. The Smalley load with 32 grains of Benchmark and 40-grain Hornady bullets was 4,189 with a 1:12 twist barrel.
Accuracy actually was improved with the 32-grain charge of Benchmark, but I was getting some additional pressure signs. Bolt lift was stiff, heavy ejector marks appeared on the case heads, primer cratering was becoming more pronounced, and re-chambering cases was difficult. At this point, I loaded two cases with 32.5 grains of Benchmark, still with the 40-grain Berger boat-tail. This was a half grain under the 33-grain load of Benchmark given me to get started on, but suggested to start off lower. At 32.5 grains, velocity was 4,193 and 4,227 fps, and here I drew the line. Bolt lift was very difficult. Accuracy for the two shots was at 0.600-inch.
By Smalley had stated he could blow up Berger bullets if pushed too hard, but they were the most accurate of those he tested.
With the three Berger 40-grain loads completed, I went to the 39-grain Sierra BlitzKing and the 40-grain Hornady V-Max, along with 32 grains of Benchmark powder. The 39-grain Sierras grouped just under 0.800-inch but the 40-grain Hornady bullets were quite wild at more than 1.5-inch. Velocity of both was close to the Berger 40-grain bullet. Bolt lift was difficult with both bullets, heavy case head ejector marks showed on both, and there were even a few cratered and blown primers on the Hornady load.
With the pressure signs knocking on my door, I backed off, but I did want to know how this cartridge was reacting to what I thought should be acceptable loads. Was I seeing in-case ignition so efficient that increased pressure was the result with the elliptical shoulder and improved primer shock wave focus?
McPherson reported good performance with Sierra 39-grain bullets up to 4,200 fps. I could not even approach 4,200 fps with these bullets in the Savage barrel with 1:12 twist and stick powder. Things changed, somewhat, with Western’s TAC powder, as I shall discuss later. Bullet pull and seating depth appeared to be within normal limits. One of the two Norma cases, received from Savage along with the rifle, had shown a blown primer.
Smalley and McPherson experimented extensively with several different barrels, and rifling twists up to 1:16. All this has a significant bearing on the pressures, as well as accuracy inherent in a rifle, barrel, bullet — and not to be overlooked, the propellant combination. Some of these loads also included molybdenum-disulfide coated bullets. In November of 2005, Smalley reported to me that it looks like some bullets will not take the velocity available. He found in his testing that he had to reduce velocities some for Sierras and Bergers, even though velocities were still well above the other 20-caliber numbers. Overall, he found Bergers were most accurate. He reported that McPherson was able to get good accuracy with the 30-grain Bergers at 4,700 in his 1:16-inch twist Pac-Nor barrel. Smalley blew them up with the 1:12 twist Pac-Nor barrel at that velocity. He reported acceptable accuracy with bullet weights below 39 grains with his 1:14 twist Krieger barrel, but 40-grain Hornady bullets still keyholed at 4,350 fps. As I reported earlier in testing the 40-grain bullets, I could not even come close to the 4,300 fps in the barrel combination I had with 40-grain bullets with ensuing pressure problems already appearing at 4,100 fps.
As I neared closure of my testing and evaluation of the 5mm/35 SMc cartridge, much has been stated with regard to ignition of this cartridge design. Stick-type powders tend to burn faster in this type of cartridge design, as compared with ball powders that appear to be influenced by shoulder configuration. Then, as I neared the end of the evaluation, I included some additional tests using Western Powder’s TAC powder with the 39- to 40-grain bullets. I thought these bullets would be an ideal varmint bullet for this cartridge, especially on coyote and fox at longer ranges. After all, the 5mm/35 SMc is a varmint cartridge.
Recent reports from others testing this chambering suggest that IMR 4320 may be the top choice for use with 39- and 40-grain bullets. I have subsequently tested IMR 4320 with simply awful results. I do not know if this is a lot-to-lot issue or something else, I do know that I will not do any more work with that propellant in this chambering. MLM
Based upon my experience, using the 28-inch, 1:16 twist Pac-Nor barrel, this is ample evidence of (partial) bullet failure, due to too-rapid spin — with that barrel, I saw zero evidence of bullet failure, even with loads launching the 30-grain Berger at 4850 fps! We expect the new 32-grain Nosler BT, promised for November release, to eliminate this problem — with its heavier jacket and solid base, that bullet should function perfectly in any 12-twist barrel, at any feasible velocity — and, subsequent testing has proven that it does.
As a gradual increase in powder charges were introduced using TAC powder, first with Berger 40-grain boat-tail bullets, the following velocities were recorded: 31 grains / 3,913 fps; 31.5 grains / 3,942 fps; 32 grains / 3,962 fps; 32.5 grains / 4,040 fps; 33 grains / 4,053 fps; 33.5 grains / 4,110 fps; (grains / 4,164 fps blew primer). Still using the Berger 40-grain bullet, I fired five shots for accuracy using 33.5 grains of TAC. Velocity figures were 4,066, 4,054, 4,110, 4,097, and 4,106 fps with a 0.800-inch group. Powder was just below the junction of the case neck and shoulder — a full charge for this bullet.
I then switched to the 39-grain Sierra BlitzKing bullet, again using 33.5 grains of TAC powder. I shot two groups for accuracy and velocity comparison. The result was indeed impressive. Both groups were just over half an inch and velocity average was a very respectable 4,165 fps. There were no untoward pressure signs noted with this combination.
A point worth mentioning here involves barrel heating. Throughout testing of the 5mm/35 SMc, cartridge barrel heating was not a real factor. Most groups were five-shot strings with little time between groups. Ambient air temperatures were hovering around 30 to 40 degrees F. From my own observation by feeling the length of the entire barrel during testing, I cannot say I noticed any reduced heating with the 5mm/35 SMc cartridge. However, I noted a difference in barrel heating throughout the length of the barrel comparing ball powder and stick powder. The ball powder tends to distribute barrel heat more uniformly than stick type powder. Stick powder appears to produce heating more from the breech to mid-barrel, as it appears to create greater pressure in the chamber as well.
Cases were quite easily sized using the Wilson dies. The sizing button causes fairly snug bullet seating. Setting the shoulder back is a real task with the ellipsoidal design, however.
As I concluded my testing and evaluation of the new 5mm/35 SMc cartridge, I believe I performed a thorough assessment of it. I did not think there were significant constraints in the testing process using but one rifle — hence one barrel. But there are imposed limits here. I later plan to add this round to my already vast number of varmint and experimental cartridges, chambering it to a 1:12 twist premium barrel for one of my rifle actions of proven accuracy. I think there are enough inherent benefits in this cartridge to provide the incentive for me to do so.
Aside from the ellipsoidal configuration on the 5mm/35 SMc shoulder, it compares quite closely to two very similar wildcat cartridges in the short-fat class — namely the 20 BR and the 20 PPC, which I touched on earlier in this article. Either of these two will fire 30-grain bullets at more than 4,500 fps, and 35-grain ones at more than 4,200 fps, or 40-grain bullets at 4,000 fps. This trio of cartridges demonstrates significant velocity gains over such rounds as the 204 Ruger or 20 Tactical (perhaps bordering over-bore capacity), which includes some markedly increased velocities, as reported by Smalley and McPherson with the 5mm/35 SMc cartridge. With lighter bullets of 30-grain weight, velocities of 4,800 fps were obtained with molybdenum coated Berger bullets at near-quarter-MOA accuracy.
I will include herewith a table of many additional loadings used by Smalley and McPherson in their testing. It must be cautioned, however, that these loads were arrived at with the barrels and rifling twists they used, and under the conditions at the time of testing.
Having worked extensively with just about every viable varmint cartridge out there, my measure of a cartridge finds me making some critical, yet well-supported, comparisons and conclusions. As a rifleman-varmint hunter devoted to seeking the utmost performance from a rifle and its cartridge, I see sufficient recognizable merits to warrant a definite place for the 5mm/35 SMc cartridge in the throng of varmint cartridges already out there.