Testing the 6mm/52 SMc ™ Cartridge
at Norma Precision in Sweden
By Smalley
Friends,

 

The data and tables By refers to in this article have somehow been lost.
While I am still looking for those on my back-up hard drives, I do not believe I ever had those.
Any record By had of those were lost after his death due to unfortunate circumstances involving his Estate.
This is entirely too bad, because the data was interesting and potentially useful.
But, unless by some miracle I can find a copy here, it is evidently lost forever.

 

Sincerely,

Mic

During the month of November 2002, I took a trip to Ämotfors Sweden, to perform pressure tests on the 6mm/52 SMc cartridge at the Norma Precision AB test facility.  By a strange coincidence, that period just happened to be the height of the moose-hunting season in the vicinity of Ämotfors.  Torbjorn Lindskog, the CEO of Norma was kind enough to allow me to hunt on the Norma hunting properties with their target/competition sales distributors one weekend of the hunt.  (More on that later.) I also had the opportunity to tour the Nexplo (Bofors) powder plant at Orebro where Alliant and Norma canister smokeless powders are manufactured.

Torb and Christer Larsson, Chief Ballistician at Norma have been very generous and supportive of McPherson’s and my efforts to develop the SMc line of cartridges.  They are excited about the potential market.  Christer worked very hard, side by side with me, to perform these tests, along with Jan-Aki Hult, and several other test technicians.  They were all wonderful, providing the help, knowledge, facilities and support for this test project.

Days were short that time of year in the Varmland district of Sweden with sunrise at about 9 am and sunset at 3:30 p.m.  The scenery was fantastic as shown in this picture of sunrise, figure 1, taken from Christer’s office desk.  I have to say it was really, REALLY, cold.  I do not think it ever got up to freezing while I was there.  It snowed twice, total depth about 3 feet.  They said it got cold after I left, the coldest in the last 30 years.  So much for global warming.  Each parking place in the Norma Manufacturing facility parking lot has a hookup for car engine block and interior heaters.  I stayed at a small hotel in Arvika, a town of about 25,000, 23 kilometers from the Norma plant.  The people there were wonderful, so very friendly, the food was excellent and the beer may be the best in the world!

The Test Cartridges

The 6mm/52 SMc cartridge was specifically designed for this test.  It was designed to have the same internal volume as the 6mm Remington, that cartridge being one of the most efficient large capacity 6mm commercial cartridges available.  We did everything we could think of to make the test conditions the same for both cartridges.  The internal volume of the test cases, 6mm Rem and 6/52, shown in figure 2, was equal within 1/4 grain of water, which is less than normal manufacturing lot-to-lot variation.  However, the 6mm/52 was about 0.4-inch shorter.  We had Dave Kiff of Pacific Tool and Gauge (a Precision Shooting advertiser) provide a chamber reamer of each; type, both having the same neck, throat, leade and pilot dimensions.

We obtained two stainless-steel 6mm barrels from Border Barrels in England and had them shipped to the Norma facility, as it is now very difficult to ship gun parts in and out of the US.  These barrels were as alike as possible in every respect.  1.25-inch diameter cylindrical, same 8.5-inch twist, same length, and same raw material lot, and manufactured in sequence.  They were first chambered to ’96 Swede Mauser actions and broken in together.  After a 50 shot break-in with a lot of cleaning they were converted to pressure barrels by epoxying a 4-inch diameter sleeve over the chamber end, cutting off the threads then rechambering to the same 26.5-inch length.  An AN type pressure tap hole was drilled into the center of the body of each case to match the Kistler high frequency pressure transducer as noted in figure 2.

In retrospect it would have been better to utilize conformal pressure transducers as the frequency response would have been better.  The same pressure transducer was used in both test barrels for each test series.

We also tested SMc proprietary reflective coatings for brass case inside surfaces.  These coatings have been shown to provide an increase in velocity in straight walled cases such as the 45-70 of about 8% but up to this time we have had no pressure data.  We tested them in 6x70mm Norma cases.  This is a very long slender rimmed case, as shown in figure 3; designed for European drilling (2 side by side shotgun barrels with a rifle barrel underneath) applications, great for hare, roe deer, and such small game.  Briefly those tests gave a 2.5% increase in velocity and a 10% increase in peak pressure using Norma’s factory load; 30 gr.  VihtaVuori N-160 and 90 gr.  Nosler Ballistic Tip.  No attempt was made to develop a better load for coated cases as time was too short and besides I just had to go moose hunting.

A note here; 12-14 hour days, seven days a week in the test lab are too much for this 70 year old body (and mind)!

The Test Setup

Cases used were all Norma supplied.  The 404 Jeffery cases were shipped to Buffalo Arms Inc where the case walls were thinned from about 0.023-inch to a very consistent 0.010-inch.  They were then shortened and rough formed to the SMc configuration, fire formed and necks trimmed to length and thickness.  The 6mm Rem cases were fire formed and neck turned to match the slightly tight-necked chamber.

We drilled a 0.060-inch hole in each test case, matching the transducer fitting in the test barrel.  The cases were then neck sized and loaded with test loads.  In general, five rounds were tested for each load.

A word here: Loads that do not fill the case completely are quite variable in both pressure and velocity.  If the powder charge is located at the back of the case, pressure and velocity will be higher, if located toward the front pressure and velocity will be lower.  Total effect depends upon the empty space available.  This is believed to be caused by adiabatic heating of the charge when the empty volume is pressurized at the front.  When the void is at the rear, the primer plume probably overpowers compression-heating effects.  Test technicians at Norma test labs are very familiar with this phenomenon and take precautions to assure the charge is always located at the back of the case for worst case conditions when developing loads for their loading manuals.  For best ballistic uniformity and accuracy, McPherson and I recommend a charge that completely fills the case or is slightly compressed.

The bullet bases for both cartridges were located exactly at the internal neck-to-shoulder junction for all tests.  This is done to assure the focus of the elliptical shoulder in the SMc cartridge is away from the bullet base, preventing heat loss to the bullet.  At the same time, this minimizes length of the powder column in both cartridges, which in some situations could be located in the neck, reducing efficiency.  Most of the cases lasted for 4 to 5 tests before erosion around the pressure test hole became severe.

Bullets tested were 70 gr. Sierra match, 105 gr. Lapua Scenar, 105 gr. Berger VLD moly coated, and 115 gr. Berger VLD.  Powders tested were Norma 204, MRP, MRP2, Reloader 19 and VihtaVuori N-550.  Powder burn rates were matched roughly to bullet weights.  Norma uses the NECO moly coating process on all their production target loads as they have noted a significant improvement in barrel life in their tests.

The Kistler pressure transducers were provided with a filter and filled with silicone grease to protect them from hot gases.  This probably adversely affected the frequency response but Norma does not use shock tube calibrations so system response could not be determined.  They plan to add this capability at a future date.

Velocity readings were taken with a 2-meter long radar setup located either 3 meters or 12 meters from the muzzle.  Velocities were not corrected to the muzzle but all test setups were the same for each test series of the two cartridges.  Testing was often stopped when production testing interfered.  You simply DO NOT hold up production on a line that is manufacturing 80,000 units per day! This sometimes forced us to move back and forth between test facilities.  Figure 4 shows Christer spending endless hours on the phone in support of production.

We could not record the entire pressure time integral for our tests even though it would have been highly desirable.  The pneumatic trigger operating system that sets start time on the pressure and velocity recorder was not hooked up to our test barrels.  It was tied up in production quality control tests.  Therefore all pressures recorded were simply peak pressures.  Figure 5 shows the test loading room, figure 6 shows the cleaning bench, figure 7 shows some of the thousands of pressure test barrels at Norma and figure 8 shows some of the five rail gun facilities for quality testing bullet lots.  Each production lot of match bullets is tested for group size!

Test Results

I could make little sense of the resulting data before plotting it.  Any given load in the 6/52 gave lower peak pressure and velocity, compared to the 6mm Rem.  However when the 6/52 charge was increased slightly to give the same peak pressure, velocity was much higher.  (See Graphs in figures 9, 10, 11, and 12 for effect of various loads.) This may have been due, in part, to the fact that the SMc liked RWS 5341 primers better whereas 5333 primers were used in most tests.  We were learning as we went.

Overall performance was gratifying with maximum velocities in excess of 3900, 3300 and 3100 fps with 70, 105 and 115-gr. bullets respectively.  Maximum chamber pressure was set at 65,000 psi for the 6mm/52 SMc as those cases easily withstood that pressure with repeated reloading.  Primer pockets stretched on the 6mm Rem.  when the pressure exceeded about 62,000 psi.  It should be noted that the thickness of the brass between the primer pocket and rim relief is about 41% greater on 404 Jeffery basic cases than 6mm Rem cases so these larger-diameter cases should be expected to withstand more pressure before yielding.

Just to be sure, we checked the acceptance data on the lot of 404 cases used.  These were very slightly (about 1.5%) softer than the 6mm Rem cases as determined by Vickers hardness.  Norma runs Vickers hardness as an acceptance test on every lot of cases manufactured to assure uniform rim and base strength and hardness.  Data was immediately available to us even though that lot of 404 cases was manufactured several years earlier than the 6mm Rem cases were.  Unless you have spent hundreds of hours searching for lot acceptance data in pursuit of the explanation for a rocket motor failure (or something similar) you cannot appreciate what a tribute this is to the Norma quality control system.

The most surprising aspect of the testing to me was the immense effect of primers on peak pressure and velocity.  The 6mm Rem liked RWS 5333 Magnum primers and gave the highest velocity with those.  The 6/52 gave lower peak pressures but higher velocities with the very soft RWS 5341 primers.  Pressures shot up with Winchester LRM primers to the point that the 6mm Rem primer pockets failed with loads that were acceptable with RWS primers.  We did not test the Winchester primers in the 6mm/52 SMc.

The other surprise was the overall linearity of the test data.  Pressure rise and velocity increases were almost exactly linear with increases in charge weight.  This is unusual, to say the least, with my test experience.

I also learned that [oddly] within the European community, smokeless powders are not considered double-based unless they contain about 10% nitroglycerin.  Thus Reloader and Norma powders qualify as single-base whereas VihtaVuori N-500 series powders are double-base.  The performance increase with VihtaVuori N-550 powder over Reloader 19 is noted on figure 12.

Moose Hunting

Hunting in Europe is much different than in the U.S.  It is very formalized.  Although I did not have this sort of experience when I had previously hunted in Spain, that experience may have been set up for American hunters.  In Sweden you have to pass a shooting test at 90 meters (100-yards) at a running moose replica, be authorized to own or borrow a gun, hunt with a group, and have a hunting license.

Torb loaned me a beautiful Schultz & Larson sporter in 308 Norma Magnum with a Leupold 2-7 scope from the extensive Norma arsenal.  The 308 Norma is one of my favorite big game cartridges, I own two, one with a 10-inch twist for 200 gr. And heavier slugs and one in 14-inch twist for 150 to 190 gr. Bullets.  Torb shoots a 300 Weatherby Magnum because Norma has manufactured Weatherby ammunition from almost the very beginning of Weatherby Rifles, and it was the original and only commercial manufacturer of Weatherby factory ammo for many decades.

First: The night before the hunt everyone gets acquainted, each hunter in the group introduces himself over cocktails (Good Idea!) then you are told what you are allowed to shoot; bull, cow or calf.  50,000 moose must be taken each year to control herds in Sweden.  I had to stand up and give a little explanation about the SMc concept and what I was testing at Norma to this group of very knowledgeable, mostly competition shooters.

The next morning after breakfast everyone shows up with a stool, warm clothes and a thermos of hot (hopefully) coffee.  Then after arrival at the hunting area you are given a map of the area to be hunted with numbered designated stands about 400 meters apart in a rough circle several kilometers in diameter.  The numbers for each stand were stapled to a stake or tree.  Next you draw lots for your stand.  Figure 13 shows the warm up teepee where we ate and drew lots.  You then either walk to, or are dropped off at your stand and there you stay.  There are then several long legged (Marathon runner type) Swedes with dogs who move through the center of the circle with the dogs on short leashes.  The leashes are tied to the dog handlers’ belts to help pull them through the thigh deep snow.

If one of them jumps a moose he lets his dog loose.  The dog will either run the moose or bay him up.  If the moose bays up then the dog handler gets to shoot.  If he runs then one of the hunters on stand gets to shoot (if his hands are not too numb).  Most moose are shot at ranges of less than 30 meters, the cover being quite heavy in most areas.  I guess that is why the guns are so pretty and never beat up like mine.  Most of the hunters kept their guns in their scabbards until they heard the dogs bark.  We did not get any moose and I am just as glad, as I better enjoy long-range shots.

Summary

I just know I will be back in Sweden soon.  There will be first-article inspections on the SMc cases Norma will be manufacturing for us, more tests to be run, beer to be drunk, and that big ol’ moose is still running around out in the woods.

The 6mm/52 SMc cartridge performed to expectation, giving velocities of 3900 fps with 70 gr. bullets and 3300 fps with 105 gr. bullets, even though it is close to minimum effective design capacity for this caliber.  It compared well with my 6/55, which gives over 3500 fps with 107 Sierras from the same case capacity as the 6mm-284 Winchester.  Internal case coatings also gave the expected increase in performance in straight walled cases even though optimum loads were not developed for these.