There are bullets designed to expand at low velocity on the market already. Fragmentation is just one way to dump energy into the target, controlled expansion is another. Since most of what we build won't ever be used on hard targets, then military grade ammo isn't our best choice.
77gr Poly tipped expansion bullets are reputed to work down to 1800 fps. Might look into that for 5.56 as they are already out there. As for either fragmentation or expansion, bullets can still hit their target out to 400m and it's been reported those using the MK18 do get hits and kills out to that range. It's more user skill than what hokey special ammo somebody is marketing.
The AR pistol with 10.5 barrel isn't at the huge disadvantage some make it out - just because military fragmentation is less optimal, the Navy and others went to better ammo than issue, which is the whole point of MK262 and certain other loads using commercial hunting ammo.
You are under no obligation to the Hague Convention in your personal choice of ammo used here on varmints of whatever description.