How Ammo Sensitive Is Your Rifle?

A couple of weeks ago, I questioned the conventional wisdom of buying a .308 bolt action rifle (presumably for long range precision shooting) merely because it shoots the same caliber as your battle rifle. The concern is that the lighter 147 grain NATO rounds may not do well in a bolt-action intended for shooting much heavier hunting or target rounds.

This can go the other way as well. Your 7.62 NATO defensive rifle may not reliably feed all 7.62/.308 ammo.

I've been trying to catch up on some reading, and came across an article in the November 2013 Guns & Ammo called "Reloading for .308 ARs" by James Tarr. He notes in the article that "[w]hen compared with the .223/5.56 AR-15, the basic .308 AR is much more ammunition-sensitive." And, "[i]t is actually hard to find a .308 AR that will reliably cycle every type of SAAMI-approved factory ammo...."

Tarr spoke to Bill Alexander of Alexander Arms about the issue. Alexander's explained that the AR-10 can be made to shoot a particular load very well, but you run into problems with other loads using different bullets, powders and environmental conditions. The key issue is going to be getting the proper pressure at the gas port--one that is able to provide enough force to cycle the firearm. But because the AR-10 operates at lower pressures at the gas port than AR-15s, it is more sensitive to variances in ammunition. Tarr states that manufacturers have moved the gas port up and down the length of the barrel trying to find the "sweet" spot, but leaves the reader with the impression that there probably isn't one.

Citing Jeff Hoffman of Black Hills Ammunition, Tarr reports that the biggest problem with AR-10 ammunition sensitivity is dropped primers--a timing problem resulting from the gas port being too close to the chamber. Hoffman recommended that re-loaders stay well away from using maximum loads in their rifles, and crimp the primers.

Alexander suggests in the article that the M-14/M1-A may even be more sensitive to ammunition than the AR-10.

I guess this brings us back to the point that post-TEOTWAWKI, scavenging or trading for ammo may not be as straightforward as you would think. It won't necessarily be enough just to find ammunition that is the same caliber.