XLP works with the Fortis Rev and Samson rails with no modification.
Troy Rails (Modify to rail is needed)
For troy rails the bottom of the rail needs to be modified to allow the set screws to be tightened through the rail. because the rail needs to be rotated, the install is accomplished by installing and gaping the gas block, marking your spot on the barrel, removing the block, installing the rail, and then installing your block.
For the Midwest SSK and SSG2, Midwest recommends cutting back the XLP piston spring just enough to allow the bushing to rest against their proprietary barrel nut. A Midwest barrel nut is a reversed, slightly elongated standard barrel nut. The bushing should not rest against the teeth of the SSK nut, it needs to rest against the outward facing flat edge. Other options are the barrel nut can simply be cut back in the one spot to where the bushing naturally rests (DO NOT CUT IT ANY MORE THAN NEEDED TO FIT BUSHING) or alternatively you could modify the bushing to allow it rest against the barrel nut (SEE PIC). Midwest advises you will void your warranty on the handguard if you modify the barrel nut.
XLP is .999 +/- tall from center bore
XLP is .565 +/- to the bottom from center bore
XLP is .97 +/- wide at its widest
The XLP system is designed to work with a standard barrel nut, or barrel nut of similar length. the bushing rests against the standard barrel nut about .85 from the receiver. If if a proprietary barrel nut is shorter than this, a shim or longer spring needs to be used so that the bushing does not slam into the barrel nut. if the barrel nut is longer than this, the barrel nut need to be cut back in the area that the bushing rests against so that the bushing rests around .85 so that the drive rod has a complete stroke.