If your gun won't shoot 168's with 41.5 grains IMR 4895, it won't shoot .
Ain't that the truth!
The 308 cartridge was developed with IMR 4895 as the propellant. Later, after the war, the Army decided to use ball powder, but I have not found any Logistical or Technical reason, so the on high decision must have been based post employment opportunities for the General that made the decision.
All, or most all, of the National Match ammunition used IMR 4895 as the propellant.
I recommend as first choice any of the 4895 series. That is IMR 4895, H4895, and AA2495. These are all so close in performance that I can't tell the difference over my chronograph. I know I used IMR 4895 and AA2495 when I earned the Leg points to become a Distinguished Rifleman with my M1a. The stuff shoots great in gas guns.
I would recommend starting out at 40.5 grains IMR 4895/H4895/AA2495 and work your way up to a max of 41.5 grains with a 165/168 grain bullet. You will be limited by your gas system, at some overpressure condition the bolt will be ripping the cartridge rims off, or you will be knocking the cases out with a cleaning rod. A bud of mine started off with a load around 44 grs of IMR 4895, because that was close to the Reloading Manual max. We had all sorts of fun trying to extract his fired rounds at the range. His load was simply too hot. I went back to his house and helped him set up his dies with Dillion case gages, because his rounds were too long. He had been getting away with this in his bolt gun, because of the massive cocking cams and leverage you get with a manual bolt, but long rounds were causing failures to go into battery on his AR10. He is now a happy camper, both his gas gun and bolt guns function perfectly and he shot nine deer in ten hunting trips. He is a good enough shot that he made head shot and neck shots at 300 yards. He said the entrance hole on a shoulder shot is a little hole, but the exit shot is the size of a small plate. At dinner this week, I was mad at him for not taking pictures of the head shot, he said it was too gruesome. We were making jokes about a 168 grain headache, and your brain on hollowpoints. The 308 knocks them dead!
You have to be more precise in your reloading with a gas gun than you do with a bolt rifle. Many bolt gunners think they are the greatest because they can beat their bolt closed, and the rifle will go bang. And then, they use the same block of wood to beat the bolt open. Bolt gunners can get away some pretty sloppy reloading habits, but follow the same sort of foolishness in a gas gun, and the gun will jam. Neck sizing or partial neck sizing will cause your AR10 to jam.
I recommend you buy a case gage like the ones in the picture, and set up your dies with these gages. For a gas gun, set the shoulder back at least 0.003" from the chamber, or if you don't know the chamber headspace, size the case to gage minimum. The first gage is a typical Wilson gage, it measures base to shoulder. The gage is cut large between shoulder and base. This gage is less expensive than the Sheridan gage.
I love gadgets and this Sheridan gage is a great gadget. It was cut with a SAAMI minimum chamber reamer. As you can see, it has a window. Unlike the Wilson gage which only measures cartridge "headspace", this gage will also tell you if your cases are too fat. I use small base dies for my gas guns because standard sizing dies sometimes leave the case too fat and that will cause feed and extraction issues.
