I've shot some of the ZQI 7.62 ammo you get at Wal-Mart out of my new AR10,
Gas guns are more finicky about ammunition than bolt guns. My advice for reloading for an AR10 is
1. Full length resize in a small base die
2. Trim cases
3. Clean primer pockets, ream to depth
4. Prime all cases by hand, verify that all primers are below the case head, and use the least sensitive primers you can find.
5. Use IMR4895/AA2495/H4895 powders.
6. Seat the bullets to magazine depth, no longer than 2.8 for the 308, shorter is fine.
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 need. But in the July 2012 issue of Guns Magazine, he is testing an M1a and a AR10 and his reloads are too tight. I find it humorous to read of him beating the bolts open with scrap lumber. Ha, Ha.
If you attempt to small base size with a spray on lube you will stick the case in the die. I recommend RCBS water soluble or Imperial Sizing wax. These are excellent lubes.
For these rifles it is safety critical to ensure that all primers are below the case head. Reaming primer pockets to depth is a good idea. Seat the primers by hand, and verify that all of the primers are below the case head. There is a chance that a cocked primer, with the anvil firmly seated on something, will cause a primer initiated slamfire. One poster swaged his primer pockets, which shaved brass donuts into the pocket. He left the donuts in the pocket, which resulted in high primers, and his AR10 slamfired in battery. Clean those pockets! A high primer can cause a slamfire but only if the anvil is firmly seated. High primers are one of the most common cause of misfires because the primer won't fire unless the anvil is seated and is pushed up into the primer cake. This is worth reading:
Mysteries and Misconceptions of the All Important Primer
http://www.shootingtimes.com/2011/01/04/ammunition_st_mamotaip_200909/
However, given a shallow pocket it is theoretically possible that high primers could slamfire, given debris in the pocket, you can get a slamfire.
Because SKS’s slamfire so often, there are lots of slamfire reports with SKS’s, Murray’s has a firing pin modification to reduce the chance of slamfires. Also, the SKS boards has this excellent “A primer on primers”
http://www.sksboards.com/smf/index.php?topic=56422.0
Federal primers are the most sensitive primer on the market and the most "slamfiring" primer in Garands/M1a's. I have lots of web accounts of slamfires with Federal primers so I don’t recommend them in gas guns. I recommend CCI #34's and Tula7.62 primers as they considered "Mil Spec" primers. Which means they are less sensitive than commercial primers, federal being the most sensitive commercial primer on the market
Use powders that are close to IMR 4895 in burning rate or just use IMR 4895. High port pressures are the concern. High pressures will create excessive bolt open acceleration. The NRA used to recommend powders that have burning rates between IMR3031 and IMR4320. This includes powders such as IMR 4064, and Winchester 748. I believe the first choice of powders are IMR 4985, H4895, AA2495. In the 308 I have used a load of 168 grain Sierra Match, 41.0 to 41.5 grains IMR4895, LC cases, and CCI #34 primers OAL 2.800 for years.
Don’t go to the reloading manual and use the maximum loads listed in there for your gas gun. You do not want to be anywhere near maximum loads. Your rifle will be ripping the case heads off your rounds. You are limited by your gas system and generally speaking, don’t push a 150 faster than 2650 fps, a 168 faster than 2625 fps, and a 175 faster than 2575 fps. Slower is fine. My velocities are from 22” barrels, if you are shooting a 20” AR10, the velocities should be about 50 fps less . Function is more important in this mechanism than speed. So, as a general rule, cut your loads. Test over a chronograph.
A bud of mine went straight to the maximum loads of the manual with his AR 10 and I was out at the range helping knock cases out of his gun with a cleaning rod. I got him to use 41.5 grains AA2520 with a 168 hunting bullet and he has been one happy camper. He has been getting head shots at 300 yards and it kills the deer dead. He won’t show me the picture of the deer with its brain on hollowpoints, but the 308 knocked it dead, dead, dead.
Check cartridges for case head separation. Gas guns are hard on brass: the bolt unlocks while there is still significant chamber pressure. Because of this the case gets stretched on extraction. Carefully inspect cases for stretch ring marks at five reloads. They occur about .4” of an inch ahead of the base. You can verify if the cases are internally necking by inserting a bent paperclip in the case, and feeling for an edge. The useful lifetime of a case is determined by case head separations, case neck splitting, or primer pocket enlargement. When any one of these failure mechanisms happens to a case, it has exceeded its operational lifetime. In my experience, US military brass holds up better than commercial cases. But this is a broad generalization. You want to use heavy cases over light cases.
I full length resize for function. For a gas gun you must full length resize your case. I recommend small base dies. The set up instructions given with sizing dies assume a lot of things. And seldom following the instructions do you ever get the brass sized to the correct length. If possible you would like to set back the case shoulder about .003” from chamber headspace. In no circumstance do you ever want to size it more than .006” as you are likely to get a case head separation on the next firing. Just take the instructions that the factory sends with their dies, and toss out the part that tells you to size to the shellholder. Or shellholder plus a ¼ turn. You will find that such guidance is inaccurate at best, rubbish on the average.
To properly size cases to a correct length you will need a new piece of equipment that has not been mentioned: case gauges. I really like the Wilson type case gauge. You size your round and drop it in the gauge. This gauge measures the distance between shoulder and base. It is a "go" and "no Go" gauge. And it is a true measurement, as I have dropped my chamber headspace gauges in my wilson gauges and found perfect agreement between them. You want to size your case between “go” and “no go”, and for my rifles, for my gas guns I size everything to gauge minimum.
This web site is really useful for showing how to use case gauges. I recommend looking at the pictures, and it explains the special case gauges needed for the belted cartridges.
http://www.realguns.com/Commentary/comar46.htm
The midsection of the Wilson gauge is cut big. It only measures headspace. What the Wilson type gauge and the other functionally equivalent gauges do not measure is "fatness". This is an important measurement for gas guns and should be controlled. You do not want any resistance to bolt closure due to overlong cases or over fat cases. It is a safety consideration for gas rifles with free floating firing pins, it is a reliability consideration for the other few actions.
I recently purchased this Sheridan cartridge headspace gauge and I think it is neat with the port and all. Importantly, it is cut with a minimum SAAMI reamer so the thing is basically, a rifle chamber. If your cases are too fat or too long, this gauge will tell you if you have an interference fit.
