Generalities of the more common AR-15 chambers and a discussion of “encyclopedic error.”
by Joe Carlos
Continuing with common AR-15 dimensions, I suspect I’ll ruffle some of my fellow gunsmith’s feathers in the section on “encyclopedic error.” We will start on the short end of the spectrum and end up on the long side.
The SAAMI chamber is old but that doesn’t mean it isn’t applicable to certain AR-15 projects today. When the .223 Remington first came out it was used mostly for varmint hunting in bolt guns. The long heavy bullets of today hadn’t been invented yet and most of the ammo was of a length to fit in the integral magazines of bolt action rifles. Because the chamber was designed to accommodate short ammo, the leade is short. I don’t normally build varmint rifles and don’t have a SAAMI reamer but references say it measures about 2.42” with an 80 grain Sierra MatchKing bullet. Shorter chambers tend to pressure up more and the conventional wisdom is to not use the SAAMI chamber with military 5.56mm ammo as it is loaded hotter than some civilian ammo. ARs have recently become quite popular for varmint hunting and if the customer plans to use light short bullets this could be a very good chamber. You should remind your customer to check the game regulations for the state where he plans to hunt as semi auto rifles are not legal everywhere for hunting. My home state, Pennsylvania, does not allow them. Of course we don’t have prairie dogs either but we do have woodchucks.
Quite some time ago when NRA bullseye shooters started to make the switch from the old M14/M1A, some ARs were chambered with SAAMI reamers. As bullets for the AR got longer and heavier, shooters with such chambers found they had to seat bullets further down in the case. This caused pressure problems and lots of blown primers. Powder charges had to be cut to compensate, reducing velocities and shooter happiness. A fair number of disgruntled shooters threw these barrels out, which was wasteful as judicious use of a throater reamer likely would have fixed the problems. Some had their barrels completely rechambered and that also worked. If you get a customer complaining that an old “safe queen” from back in the day is blowing primers, you might want to take a quick Stoney Point reading and determine if it has one of these short chambers.
A chamber popular with Service Rifle shooters is Compass Lake Engineering’s proprietary chamber designed by Frank White, and it’s a good one. Overall cartridge length, measured with the Stoney Point using an 80 grain SMK is 2.450”. Derrick Martin at Accuracy Speaks has a similar reamer resulting in a chamber not much different at 2.442”. For the Across The Course Service Rifle shooter these chambers are my top picks. By starting a bit on the short side these chambers offer stellar results with magazine length ammo while offering hundreds, if not thousands, of rounds of additional service life. There will be a small reduction in powder charge with these chambers when shooting 80s compared to the next chamber.
Read more in our August 2014 issue. Back issues are available.