Do We Need a 5mm (20-Caliber)
Varminting Round?
M.L. (Mic) McPherson: December 2004

Synopsis: As the former editor of Cartridges of the World, I may be in a somewhat unusual position to recognize that, in general, we have far more chamberings than we could ever need.  Beyond some very specialized applications, there exist precious few holes in the lineup of readily available factory chamberings.  However, such holes do exist.  Here we will explore one I feel is significant.

The sport of Varmint Hunting has expanded quite significantly in the past twenty or so years as ranchers and farmers have grown to appreciate that sport shooters can be their ally in controlling unwanted, dangerous and destructive pests about farm and ranch.  Without getting into a deep discussion on this subject, I can simply report that all unbiased and honest experts in this field agree that varmint hunting is by far the most humane control method for such pests as the short tailed ground squirrel species that Lewis and Clark unfortunately named the Prairie Dog.  I prefer the more appropriate moniker, Prairie Rat.

These critters have no effective natural enemies and the natural consequence is that they destroy their habitat as they multiply and must therefore spread out in a circular pattern, leaving a barren, scarred moonscape in their passing wake.  It can take decades for such lands to fully recover before another cycle can resume.  However, in much of the US before this can happen, the bubonic plague will devastate entire regions.  This is almost inevitable whenever population density of a region exceeds a certain threshold.  On tilled and ranched lands, long before either of the above scenarios can play out, the land owner usually opts to control these critters and limit their destructive force through authorized poisoning, which is expensive but can keep the population in check.

Alternatively, the landowner can encourage varmint hunting, which if practiced with sufficient diligence, can keep the population in check at a sustainable level that the landowner can afford.  Contrary as it may sound, as experts in this field agree, varmint hunting has proven to be the best method of population control for such species.

Existing Equipment

When shooting conditions are good, with a modern factory varmint rifle, good factory varmint ammunition (Black Hills Ammunition is rightly renowned in this field), a good scope, a laser rangefinder, and the proper rest, a practiced marksman can expect to routinely center a prairie rat to at least 300 yards without much problem.  Most who have been at it a while can extend this to about 400 yards.

Similarly, most who have sent enough bullets down range in this endeavor will agree that on most days, field conditions and other limitations make it very difficult indeed to center targets much beyond about 400 yards.  Therefore, most of us will agree that a varmint chambering that will deliver explosive performance to 400 yards will satisfy the vast majority of our varminting needs.  So, what is the ideal gun and chambering to achieve this goal?

Certainly, any high performance 6mm chambering will deliver a properly constructed light bullet well past 400 yards with sufficient velocity to result in explosive terminal performance, which leaves no question as to the instant result of a decently centered hit.  However, such guns tend to generate so much recoil that we cannot see the impact unless the target is near the limit of our ability to make such hits.  This means that on close-in targets we miss seeing the impact, which is what this game is all about.  Moreover, such guns generate so much barrel heating that use is limited — when the shooting is good, we must frequently stop, to allow the barrel to cool!

For these reasons, many serious varminters opt for high-performance 22s.  The Swift is certainly up to the task, as is the 22-250.  However, these numbers also generate sufficient recoil to prevent the shooter from seeing impacts on close-in targets and barrel heating is still an issue.  With best handloads, the 223 may be the best choice for most applications owing to reduced recoil (one can see hits at significantly closer distances) and significantly less barrel heating (one can fire about twice as many quick shots without overheating the barrel).  The limitation is, the 223 simply cannot deliver explosive performance at 400 yards.

Modern Alternative

I believe consideration of the above can give one a reasonable understanding of why the 204 Ruger came to be.  With best bullets, it shoots usefully flatter than the 223 and delivers similar performance at long range with similar barrel heating and usually somewhat less recoil.  Unfortunately, the 204 simply cannot deliver explosive performance at 400 yards.

Higher performance 20-caliber numbers have significant merit.  Consider the 5/35 SMc ™ (5mm-bore/35 grains usable case capacity — grains of water to base of neck).  This cartridge utilizes patented case design technology — ratio of case diameter to bullet diameter and shoulder design are chosen to provide maximum velocity with minimum barrel heating and wear.  The superior design allows the 5/35 to achieve a significant velocity edge over the 204 Ruger — about 300-fps with any given bullet — despite essentially identical usable capacity.  Superior case design allows the 5/35 to achieve such performance without producing significantly more barrel heating than either the 223 or the 204 (see graph).

The 5/35 SMc is fully capable of delivering explosive velocity to targets out to about 400 yards and it will do so while generating much less felt recoil (sight picture disturbance) than will any 22-caliber with similar capability.  In recent field testing, three shooters compared two almost identical guns (Savage Low Profile Varmint Models fitted with the Leupold VX-III, 8.5-25).  One was a factory gun chambered in 223; the other was a factory Custom Shop gun chambered in 5/35.  All three shooters agreed that, with best handloads, it was possible to see impacts on significantly closer targets when shooting the 5/35 SMc, despite the significant ballistic advantage it offers.  Similarly, when the shooting was good, the 5/35 did not heat the barrel any faster than did the 223.  Subsequent barrel heating studies carried out with the help of Savage Arms proved this to be the case — with best loads, barrel heating in the 223, 204 and 5/35 is similar (see graph).

Current Limitations

Currently, the 5mm bore suffers from inadequate development.  For example, most of the bullets available are marginal or sub-marginal for use with full-power 5/35 SMc loads in a 1/12 twist barrel (default standard for 5mm), which is necessary to stabilize 40-grain bullets.  Heavier bullet jackets are needed.  In my humble opinion, the real issue is perception.  The 5mm should not be considered a long-range varminting option.  For this reason, the 40-grain bullet is unnecessary.

Had Hornady standardized a 35-grain bullet as the heavy option for the 5mm bore, they could have chosen a 1/14 twist and we would then find that all 30, 32, and 35 grain bullets now offered would work fine, even with the hottest loads in the 5/35.  Bullet makers will get this sorted out.  When Nosler offers a BT at 30-grains (or less!) we will have a bullet that will really bring the 5mm to the forefront, I am sure.  Meanwhile, I am using a 1/16 twist Pac-Nor barrel on my Savage Custom Shop 5/35 SMc.  Ballistics and accuracy with the 32-grain Sierra BK are impressive, to say the least.  See associated ballistics graphs.  I can suggest that anyone planning a custom 5mm might want to consider a 1/14 twist for use with light bullets.  (Subsequent testing at lower elevations proved the 1:16 twist to be inadequate for tipped 32-grain bullets.)

Recoil is so modest that I can literally see the paper slap when the bullet hits it at 100 yards! I have modified the stock on this gun to reduce muzzle climb (I changed the taper angle on the forearm and the buttstock), which helped in this regard — the gun hardly moves anyway.  This is my new (high-elevation) varminting combination.  When one considers the ballistics, it is hard to see what could be better: BC = 0.22; velocity = 4650 fps.





Obviously, in my prejudiced opinion, the 5mm has a useful place in varmint hunting.  Those interested in pursuing premium 5mm cartridge performance can order a 5/35 SMc chambered rifle directly from the Savage Custom Shop: 1-800-370-0712.  Cases are a simple conversion from 6mm Norma BR (22 BR followed by 5/35 FL sizing).  Custom Lee Dies are available from me.  Finished cases with the proper headstamp are forthcoming from Norma (these are no longer available).