I have a .270 Win H&R that has an oversize bore, so Larry's suggestion is a good one to check. If the 85gr loads are tumbling, there's a problems with the bore or maybe a bad crown, sounds like a problem for H&R to solve if you don't want to try the crown touch up in the FAQs. I think the Amax is a 105gr bullet, Hornady doesn't list a 100gr, it may not stabilize in the 1:10" twist barrel, and there have been some unsubstantiated reports of slower twists in H&R 243 barrels. I'd be contacting H&R and make arrangements to have it returned if you can't get it to shoot 2" or under at 100yds with factory ammo, ask them to cover return shipping too, toll free (866)776-9292.
Tim