>>64159304 (OP)
If you believe in bullpup's advantages the buffer tube is incompatible with that or a folding stock.
The forward assist is pretty much useless and could probably just be removed to simplify the design. (Original design didn't have it, Eugene Stoner knew you could clear all malfunctions without it, and it was only later introduced due to an arbitrary requirement by the Army contract)
AR-15s also have a relatively short limit on cartridge length (and the part compatibility between AR10s are much worse so you don't see as many). You could make a case for quite a few cartridges of similar size that have better ballistic performance than 5.56 just by virtue of slightly longer bullets that AR15s struggle to accommodate.
Assuming you are talking about civilian AR15s and not M4/M16 designs as a whole since you're not concerned with full auto fire, you could probably pack a slightly stronger cartridge as the "standard" version instead of .556 (6.5 creed? idk?). .308 is pretty abysmal for full auto and led the army to .556 but I think you could find some nice semi auto cartridge that falls somewhere between .556 and .308. Still low recoil but maybe a bit more power to be more useful than an ar15 for hunting and such. But most people just want a cheap round to shoot so .556 is hard to beat in that respect.
