I run H buffers in my middies and have run H2 buffers in my carbines without issue. Set your gun up for 5.56 pressured ammo and the H3 will probably work fine. Throw in some Rem UMC 55gr FMJ and it will probably choke.
It all depends on what ammo the gun is setup for. My first Noveske 18" SPR barrel is an intermediate gas. I figured a standard carbine buffer would be the one to use. I ran some XM193 ammo through it only to have it cycle so fast it would skip stripping a shell out of the magazine toward the bottom of the mag. I called Noveske and was told their barrels are setup to run MK262 ammo in 120 degree heat, ie., lots of pressure. A stuck an H buffer in their as they requested and it ran fine.
Each gun is it's own, a 223WSSM I worked with, needed a 9mm buffer and an extra power spring or it would pull the rims. The guy stuck a Tubbs weight system in his carrier and a Tubbs spring(because someone told him it was what he needed) and had to drop back to a carbine buffer. It was all my fault of course...
