Hi, again Markd, thanks texnekkid for post,(retro, I think he means old like me, heh, heh).
A word of caution, in some combinations of barrels and receivers the barrel extension will bind on left shell stop. Put it together with bolt back and with springs and friction pieces off, by sliding barrel in and out you can tell if it binds and observe operation of shell stop. Rarely do you need to adjust clearance, then only a few thousanths between ramps,(fitting barrel to receiver rather than changing shell stop lets you use other barrels as before). You can feel when stop ramps up on extension and tell if it is binding or not, twist barrel back and forth with finger on shell stop, if you feel any movement at all , you're good to go shoot a train load of shells. Also check clearance between barrel and forearm with barrel in battery position, if you have a little binding, it could be here, a little scraping is all that's needed, some contact and rubbing is normal. Some barrel extensions have a machined groove between ramps, some do not, you can compare with other one if you can. I hope to be helpful and not scare you, this is not very critical. I dont know your level of mechanical skill, if you need help contact me.