Runaway M1 Thompson - 22 conversion

[Grasshopper]

Hi All,

 

Any thoughts?

 

Before I bother Merle, I'd like to throw this one out there to see if anyone has had a similar problem. I've been running one of Merle's 22 kits for about 8 years now. In the past it has run great. Now it runs non-stop. Really. Non-stop. 1 pull of the trigger can mean anything from 1 round to 20 rounds. Never a miss-feed.

 

I've checked and the cocking handle is not getting back far enough to get the bolt back onto the sear. (Put my finger on the side with the cocking slot and moved forward until it just touched when running.) I've gone through and cleaned everything from muzzle to guide rod and it has the same issue. I've even used same lot ammo as before and it just runs away. I've clipped the recoil spring 1 loop at a time until it would not fire and it still ran away. I've change recoil springs. Extractor is not rubbing. Ejector is clear of everything. No 45 ejector is in the gun. W/O the recoil spring bolt motion is a smooth as glass.

 

There are a few things I've noticed. The firing pin strike, while evident, doesn't crush the rim of the case. The rear of the case has a dent but the rim outer edge radius is intact. Could the hammer/firing pin be worn? Also the cases are getting a bit scratched on the outside. I did run some "mature" Federal Bulk Pack with a bit of white fuzz on the bullet. Could this have scratched the chamber preventing the case from sliding thus reducing the push on the bolt? Also it seems the cyclic increased a bit. The gun would run on any ammo from standard velocity on up. Now it runs away on Fed Bulk and higher power 40gr ammo.

 

I've got video of the gun from years back and the cocking knob was going back quite a bit more than now when running. Sounds like friction someplace but I'm at a loss.

 

As mentioned, this kit has been a crowd pleaser for years. It has well North of 30k rounds through it.

 

I've pulled back the cocking knob and dropped the bolt w/o pulling the trigger and the bolt doesn't skip over the sear. The bolt is not getting back to the sear.

 

TIA,

 

Grasshopper

Edited May 15, 2017 by Grasshopper

[Tombstone Canyon]

Sounds like your main spring has lost tension over the 8 years you have been shooting with it. I don't have any direct experience with your particular gun but I do know that on guns that have variable main spring tension (Lewis Gun) you can set the spring so that the bolt carrier never quite rebounds to the sear. This results in a runaway situation as you describe for your Thombson. Another sign of a weak spring is the lack of deep primer strikes, as you mentioned as well. Hope this helps.

[full auto 45]

Have you tore down the lower and cleaned it? Have you run a boresnake through the barrel a couple of times? Just a thought, but you might try to break it all down, throw it in an ultra sonic and give it a full on spa treatment.

[TSMGguy]

I'd try a different high velocity ammo with a heavier bullet before doing much else.

 

Had much the same problem with the .22 kit in my Vector Uzi. Switching to Remington "golden bullet" .22 solved the problem.

[Grasshopper]

Hi All,

 

I spent some time on the phone with Merle and he made several suggestions. The one that fixed the problem was cleaning the chamber. Evidently there was something rough in the chamber. I ran a cleaning rod in from the muzzle end with a slotted tip on the breach end. (Did that as the ejector makes it hard to clean from the breach end.) A small piece of Scotch-Brite in the slotted tip put in place at the breach end and pulled the tip into the chamber about 20 turns. That did the trick. Dribbling extraction went to empty cases being thrown 6' away. The gun runs on anything as before.

 

I think I'll try this on some of my other conversions.

 

Thanks for all the suggestions,

 

Grasshopper

Edited May 17, 2017 by Grasshopper

[timkel]

Interesting thread. I have encountered dirty/rough/tight chambers before in 5.56 and 5.45, but not 22lr. Makes a lot of sence as how dirty 22lr runs.

[Adg105200]

I've had that in my 10/22 where it was dirty enough that the extractor slipped off the rim of the case. I found that as far as solvents go Montana X-treme .22 solvent works the best, but it's not cheap. As mentioned above I would strup it down and give it very thorough cleaning.

 

Andrew