Hard to say, my cousin swapped 2 or 3 of his 24" barreled uppers to carbine stocks and besides his 223WSSM, he had no issues with standard carbine buffers. But never tried one on a 20" barrel.
VLTOR/BCM developed the A5 stock for the Marine Corp. It has been said that a carbine stock on a rifle barrel doesn't work 100% of the time (although Canada's C7/C8 has been doing this for years) and this new stock cures the problems.
