The Green spring is suitable for A1/A2 fixed stock rifles with rifle length buffers and is optimized for lesser calibers like .223, 5.56, and others.
It can also be used in VLTOR A5 buffer tubes and may be appropriate for heavier calibers with long barrels when combined with H2 or H3 carbine buffers.
For over gassed heavier caliber guns with rifle length tubes, SPRINCO USA recommends their Red or Orange carbine springs with a standard carbine buffer and spacer.