Numrich barrel extension
I have purchased the barrel extension from Numrich. It appears that they took an old upper, cut off the barrel and cored out the spot where it used to be, and then fastened on a new barrel. Bore is perfect. Bolt is unblued, though that's probably a matter of taste. Two problems:
-1- The frame has a "step" just below the barrel, and the old barrel extension has a notch there. I don't know whether this serves any purpose other than preventing barrels without such a notch from being used on that frame. If not, it shouldn't be too hard to grind it off, but I'd like some confirmation that it's safe to do so before I start taking off material.
-2- The old bolt has a 'bulge' on the bottom which serves to fully cock the hammer when the bolt is drawn back manually or 'automatically'. The new bolt lacks this bulge. When I test the new bolt in the old upper (can't test it in new upper, due to problem #1) manual cycling does not cock the hammer; rather, the hammer ends up caught in one of the notches on the underside of the bolt. Is this normal (automatic cycling would probably put enough momentum into the hammer to carry it past the cocking point)?
BTW, with regard to the notion of using the old (nicely-blued) bolt in the new upper, there are two problems: -1- The old bolt's firing-pin hole is too big, and so firing pins will not last long at all with the old bolt; -2- the bulge on the underside of the old bolt does not quite fit the new upper.