Direct from Geissele:
Thank you for your interest in our products and for contacting us. The Super 42 Spring and Geissele buffer are designed to work together as a set; the spring will not fit over a standard buffer. If installed with a standard buffer damage to the rifle or receiver extension will occur.
The Super 42 is compatible with carbine length buffer tubes.
The Geissele Super 42 Spring and Buffer Combination ships in the standard H1 configuration. The H1 and Braided Wire Spring works well in 18” SPRs, 16” rifles with mid-length or carbine gas systems and some 14.5” rifles. Most 14.5” rifles would benefit from the H2 Buffer.
Weapons with barrels shorter than 14.5” will need some testing with H2 or H3 buffer combinations to prove out which is most reliable. We sell additional Tungsten weights to convert the H1 all the way to an H3 if needed.