I just tried something that worked out pretty well.
This is on a J&T 300-321 16" upper. I was getting about 2" groups or so out of it at 100 yards with subsonics (Hornady 208 AMaxes and SMK 220s).
I've been meaning to try lapping the bore to see if I could improve that.
I finally got around to trying something. I bought one of the 30 caliber FinalFinish kits. It comes with 50 175gr bullets, 10 bullets in each grit compound.
I loaded them up with 13gr of AA1680 and followed the instructions (clean well between each 10 rounds). After firing the 50th round I gave it a good cleaning and started firing the load that it seems to like best: 11.0gr of AA1680 and a 208gr AMax at 2.170 OAL. I'm getting just under 1100 fps with that load.
Check out the before and after (same 208 AMax load, suppressed):
That is two rounds at the bottom of the X and five in each of the groups adjacent to the dime.
Saying that I'm extremely pleased would be an understatement. I was about to give up on getting a 300-221 to shoot decent groups with subsonic ammo.
The bore is almost a mirror polish now. It was grey before and I'm wondering if some remnant of the parkerizing process is making it into the barrels during production. This is the fourth cheapie 300-221 that I've run into that wouldn't group well at 100 yards w/subs.
I may run the same process on a .308 Savage Scout rifle that doesn't want to shoot under two inches at 100.
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