The best way to get better accuracy from the Barrett would be to hand load them yourself.

That would also mean that you'll have to load each round one at a time into the rifle. Keeping AMAX stuffed into a magazine isn't the best for accuracy. That'll mean loading them a little longer then for magazine length.

You should be able to get MOA out of the M82 with custom made hand loads. Maybe one of the other owners here can share their load info.

I think it was K&P barrels that used to do work on M82's to make them more accurate. You can try contacting them and see if they'll share some info with you.