I hope his chamber does not need polishing.
Even with military semi autos, they were fed new ammunition made to Army specs, which are the same dimensions as SAAMI specs. And that was for function. Most of my military rifles do have larger chambers than commercial, but even then, standard sizing dies may not reduce the brass enough to create clearance chambering and extracting. A round may barely chamber, because it is over sized, but stick on extraction. Small base dies are supposed to restore cases to "factory dimensions", but even then, brass may have expanded so much, after fired in huge chambers, that a small base die cannot size the case down enough. This is why expensive Roll Sizers are on the market. This is one example:
http://www.casepro100.com/
And roll sizers would not be on the market if experienced shooters had not found they needed them for function reliability.
There are some who say small base dies are not needed in these rifles, the Gunwriter Mike Venturino has been one. For years he has been saying in print that only standard sizing dies are needed for military gas guns. But in the July 2012 issue of Guns Magazine, he is testing an 308 M1a and a AR10 and his reloads are too tight. After reading his pontificating that small base dies were not needed, I found it humorous to find him beating the bolts open with scrap lumber. At least he was honest in his experience, and maybe he learned something.
Let me tell you one of the most dangerous conditions in the Garand mechanism. Ammunition fired in a military chamber, then sized with a standard sizing die, primed with Federal primers, and fired in a match barrel in a Garand/M1a. The firing pin is free floating in a Garand type mechanism and there is no effective firing pin block to prevent the firing pin from rebounding off the primer. The firing pin is traveling its fastest just at cam down. If there is resistance to chambering, such as a fat, or over long case, that firing pin is rebounding off the primer, and the lugs are not in engagement. And if the primer ignites with the lugs out of engagement, that has hurt a number of people, and rifles.
One shooting bud wanted an accurate Garand to compete in Garand matches. He had a Wilson match barrel installed, used an old 30-06 standard sizing die he had been given. He used brass given to him, or brass he fired in the military barrel. May have been a combination of both. He did use an old 30-06 sizing die he had been given. He did not use cartridge headspace gauges to set up the dies. He sized to the shell holder plus a quarter turn. This practice is nuts for gas guns, and might not have sized his brass below SAAMI minimum length and diameter. Bud also used Federal primers and went out to zero his new match Garand. The action slamfired out of battery and blew off the receiver heel. I pulled targets with Bud, and his comment was
"people ought to know how dangerous these things are to reload for".
Federal primers are the most sensitive primers on the market, and are the most slamfiring primer out there. Though there are slamfire accounts with all primers, mil spec and commercial, but Federals lead the pack. In fact, Federal used to brag that they made the most sensitive primer on the market. And they still do. Luckily, AR10's tend to slamfire in battery, but there is the chance of an out of battery slamfire in the things. Very, very rare when it happens. It has to do with bolt carrier bounce and a slight delay to ignition. The bolt carrier will bounce and turn the lugs out of battery. If a sensitive primer with a millisecond or so dwell ignites when tapped by the firing pin, the lugs might be out of battery.
