The Pac-Lite upper on my Ruger Mk. II .22 pistol is a source of much joy, a big improvement for the little target handgun -- except for the small machine screw that holds the ejector to the upper.
It has gotten loose in the past and been tightened, but this time when the gun started having feeding and ejection problems, I took it apart (paperclip, toolmaker's hammer, brass punch, mild cursing) to find the screw gone! Looks like a flat-head 4-40 x 3/16" and probably mild steel. I put in a binder head, brass, and will see if it runs more than a couple of rounds. (Liable to shear, which may be unfun.) Reassembled with the usual grief, wooden mallet and a lot of test-fitting. It's not that the Mk. II is all that difficult to take apart and put back together, it's just very different to most handguns and has a higher "have to hold your mouth right" quotient.
I've got a call in to Pac-Lite and I'll be investing in some Loctite once I lay hands on the proper hardware -- may have to buy a bag of 50 or more from McMaster-Carr to get decent quality if I can't shake one out of the manufacturer.
6 months ago