One way to get the 8" MPX pistol to 29" without pinning and welding a muzzle device is to get the version which comes with the side folding arm brace. Unscrew the pistol buffer from the side folding mount, screw in a 6 position buffer tube, and use a longer stock like the Magpul SL. This gets the OAL to around 29.2".
Hmmm good info