This is an anaerobic sealant that only cures when confined in the absence of air between close fitting metal surfaces. It is of medium or even low strength since you want to be able to still remove the parts. Don't try to use a high strength sealant, for that reason. Order an extra barrel strap installed close to the plastic breechblock of these guns or else tightening the intermounts will lift the barrel, causing blowby at the transfer port, and also crack the breechblock eventually. I assume the same spring will work on all the models listed in the subject line above.
Helps get the notchiness out of the pull.
Also note that the front sight moves every time you pump up, due to being part of the barrel band. The first one to be honest, just seemed to improve things, as I don't have a chrony to prove it. Oh yea get a sholder stock ,makes a fun carbine. Also information on tuning would be grateful. It's pretty accurate.