Robert Naess got the gun sorted for me and Reuben took care of the bill. Here is the assessment:
The barrel appears to be a replacement to me, but it might be original. The gun has an early serial number, 344, and it is possible that early guns had composite barrels in the fashion in which yours was done. The breech ring, which is numbered to the gun, is a separate part, bored out, threaded and a barrel threaded into the ring and cross pinned. Bore is excellent.
All the M6s, both police and commercial guns, that I have worked on have been higher serial numbers and all have had numbered, one piece barrels, so this one is unique in my experience.
The bolt in your gun is a composite also. The body of the bolt is one piece and the smaller OD section with the firing pin, extractor and ejector groove is a second piece. You might recall what it looks like. The body is half-section for part of the way and the second piece is inserted into a centered hole about in the middle of the body and cross pinned with the smaller OD part extending forward.
The bolt impacts the breech face. Since the headspace was too long, I chucked up the bolt to skim the part of the bolt that impacts the breech face. Unfortunately, the smaller OD part is no longer perfectly concentric with the bolt body interfering with working the face of the bolt that impacts the breech face. I removed the cross pin holding the smaller OD section in the body, but could not remove the smaller OD part. I was able to skim some metal but the eccentric smaller OD section interfered and not enough was removed to close up the headspace.
I removed the barrel, skimmed the breech face and tested the headspace several times to allow the small OD section of the bolt to close up the headspace sufficiently to correct it.
The extractor was pretty well worn and would not reliably extract so it has been replaced.
On removal of the barrel, I also disassembled the foregrip from the barrel. The foregrip was cracked through along the woodgrain at the thinner cross section. The mounting bar for the grip is screwed into a channel on the top of the grip which was holding it together. I glued the grip back together with an epoxy glue. Other police M6s that I've had also had cracked grips due to the way the grips are made with the diagonal woodgrain. The angle of the grain makes the grip vulnerable to cracking when the grip is pressed forward or backward in use.
The elevator for the rear sight is missing, which you might already know about. I don't know where to find one of these, but there are sight specialists who will have the parts since it is a standard KING sporting sight from that era.
I have tested the gun with several full mags and it functions as it should. Mag works fine.