I don't see how grain weight can make that much of a difference in terms of damage and whatnot. In fact, a 77 would be better as you've got less unburnt powder blasting the shit out of your main baffle.
If you're talking about overgassing your gun that's one thing, as it's understandable to not want an adjustable gas block. Buffer weighs are cheap and easy to swap tho and it's very standard to do so.
Most cans are rated higher than the cartridge they're usually used for, like a 5.56 can will be rated for 22cm or a .30 will be good to 300RUM or PRC, or some amount of full auto. Auto is gonna be tougher than most cartridges you can throw at it, as hot metal wears much faster. Sound wise I think it depends on gas volume of the cartridge if you have an undersize can but that doesn't matter when you're wearing earpro regardless.
There's videos of guys shooting .22 cans with 5.56, and although they can take up to a make or two, there's just not enough thermal mass or cooling and they get weak and pop.
Also idk how you're getting better groups with green tip unless. Either you've shot the shittest reman or you landed on some great stuff. What barrel you running? 55gn PMC Bronze and X-TAC have been the most accurate FMJ I've shot out of my guns. A number of dudes on YouTube have gotten great groups, like in the CDOES Ruger American video where he gets a 3/8 inch 4 shot group with a flier. The X-TAC is some hot stuff too, pretty damn close to NATO power.