Twenty year old story now but I still enjoy telling about it.
Most folks who know me are aware that I much prefer vintage and antique firearms. I have little interest in modern mass production guns and I'm constantly on the look-out for interesting old stuff.
I acquired a very old Colt Single Action Army .45, well worn but apparently complete and functional, with "US" frame marking. Checked the old books and determined it had been delivered to the US Army in 1882, and the serial number was within the range of subsequent modifications from the original 7.5" barrel to 5.5", referred to as the Artillery Model by many collectors.
After a good cleaning I took it to the indoor range for test firing with low-pressure .45 Colt cast bullet ammo. Loaded the cylinder, lined up the sights on a target at 25 feet, squeezed off a shot. The Colt responded by rapidly firing 3 shots, then locking up with the hammer at about half-cock position.
Hammer would not go to full-cock position or drop to the frame fully, but the cylinder would rotate freely. I carefully removed the base pin and removed the cylinder, which made the piece safe to handle and examine. What I found was that the recoil shield was missing from the frame. The hammer was jammed into the frame by spent primers, and the 3 expended cases in the cylinder had no primers in the pockets. After coaxing the spent primers out of the hammer recess the hammer again functioned normally.
Explanation: Without the recoil shield in position the primers were not supported upon firing, allowing the primers to be blown out backward, driving the hammer to the rear and rotating the cylinder to the next stop. Since the trigger was being pulled the hammer released automatically and fired the next cartridge. The cycle continued until the spent primers bound up the hammer and stopped the action.
It took about a year to locate a replacement recoil shield, then having a good gunsmith fit it and braze it into the frame. The revolver now functions properly and safely. I think the originals were a press-fit, perhaps peened into place, but I feel better having it securely brazed with silver solder.
During that year I think I was the owner of the only known example of a fully-automatic Colt Single Action Army revolver!
Lesson learned: Be much more careful in examining the old guns, if necessary physically comparing them to other examples to detect any possible damage or modifications.