In the past year or so, I've come across several Luger barrel extension ['receiver'] issues . There are probably more, but these are some of the ones I've seen. Some are bent forks, twisted forks, and misaligned forks. Any of these can affect proper functioning.
Caveat: I am not a gunsmith. I'm not an expert on Lugers. These are just a few issues I've encountered, and how to identify them. You can usually feel these issues, or even hear them. Instead of a nice smooth movement, the frame/extension/toggle interface will feel or sound gritty. Or take more effort to hand function than it should. I strip my toggle and extension when checking, as well as clean it thoroughly. I leave the parts dry, to amplify any irregularities. Then I try to identify the area causing the issue.
The tools can usually be found at any hardware or auto parts store. I'll demonstrate on an upper I got in a few weeks ago.
To start, I check to see if the forks are level.
Pic 1 shows two small spirit levels placed on the upper surfaces of the forks. The extension is positioned on a pair of ground blocks on a millimg machine table. These forks are nice and level.
Pic 2 shows the levels checking for side-to-side misalignment. I have a nice snug 8.8mm drill blank in the bore, and a 9mm squib rod in the rear to balance the extension. Happily, the forks are aligned with the bore.
(The macro setting on the digital camera exaggerates the fisheye effect and distorts the spirit bubbles).
Pic 3. I've placed a straightedge along the left/sear side of the extension. I have a bright light focused on the back of the straightedge. No light along its length; it's nice and straight.
Pic 4. The straightedge is now on the right/ejector side of the extension. You can see light in the middle section of the straightedge. This fork is bent in.
Pic 5. Using a ground half-inch block and a feeler gauge to see how much the fork is bent in. I get .006" at the rear.
Pic 6. I get a reading of .012" at the front. So this fork is bent in .006".
Six thousandths is not an awful lot. A sheet of lined school notebook paper is about five thousandths. I'm not going to try to straighten this fork. I'll just lap the fork and the frame ear it rubs against with diminishing grades of polishing compound. IMO there is too much chance of over-correcting or even fatigueing the fork. This Luger has a matched upper and a matched lower but not the same match. It's a shooter. Taking into account the clearances of the mating parts, lapping should get me nice slick surfaces and smooth functioning.
I'm sure that there are other members who have similar or additional methods of checking for barrel extension issues. I would be very interested in reading about them.
