Yes, your carbines should hold for longer distances than stated. I don't have a Henry but have heard great things about them. I want one with the heat treated finish and octagon barrel.
But as for PCC rifles, my bolt CZ .22 will keep a tight group at 100 yards. Heck, I have a HiPoint 9mm carbine that will keep a 6 inch group at 100 yards.
I would sure try another barrel on the Bushmaster. There isn't a lot to an AR. Slap on a heavier barrel, like Scap said, not a pencil thin one. Get a free float hand guard and try it again. 5.56 as Ramroo pointed out, not 223. By the way, are you shooting 5.56 ammo from a 223 barrel? If so, it could be your issue.