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M1 Bridgeport Running Away On Me


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Started doubling on semi.

 

Then would chatter to a stop when you got off the trigger on full.

 

Now she is her own master.

 

Just a sear? or sear and bolt?

 

Sorry for cell pix. Hope they work out. Engagement surfaces seem rough/burred.

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Edited by Deavis
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It certainly looks like both the bolt and the sear need to be replaced. When

you fire it on semi does the bolt stay back at full cock every time? The other

thing to consider is that the barrel is shot out and the bullets no longer tightly

seal the bore resulting in low power and short recoil. The gun doubles or runs

away because while the bolt recoils far enough to eject and feed cartridges

it does not go back far enough to be held by the sear.

 

 

Bob

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Clean the lower real good. Pull the wood and put it in an ultrasonic cleaner for a couple minutes. Could be a little crud in it. My M1 would run great on FA, but stop in semi. Pulled the wood and put it in the ultrasonic with some simple green cleaner. Dried it off and sprayed it down good with Rem Oil and it runs great now.

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I have a spare complete trigger frame with new parts in it from an IMI dummy.

 

Hopefully a local buddy will let me use his M1A1 bolt until I can get one ordered.

 

With new sear and good bolt I should be able to discern if it's the barrel but I am suspecting that it may be what Bob said. Cyclic has gotten a lot slower and chattery.

 

She needs a proper M1 barrel anyway.

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That's the most worn sear I've ever seen! I'd certainly replace it. The sear notches in the bolt are also quite worn. All of this is clear evidence that this is a gun that is much enjoyed, as it should be.

 

Just the standard reminder about correct TSMG trigger technique: we treat a Thompson's trigger like a switch, pulling fast and releasing quickly. If the trigger's squeezed like that of a target rifle, you get rapid wear to the sear and sear notch.

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Could one put this bolt in a mill and re-cut engagements?

 

I think it would result in a slight reduction in forward bolt travel during firing for the initial trigger activation but I seems like there is plenty of room to let the bolt run forward and strip a cartridge.

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Hi Deavis,

 

Just a safety thing.

 

With the gun unloaded (and make sure there isn't a round anywhere near the gun), pull back the cocking handle to retract the bolt until the handle is resting on the back of the slot.

 

Now, release the cocking handle briskly w/o pulling the trigger. The bolt should stop on the sear. That would mean the gun is safe. If it skips over the sear, you may have trouble in the future stopping the gun. I'd consider the gun needing work and prone to runaway if the bolt skips over the sear.

 

I hate to do this sear test. Seems to be hard on the parts. It normally does this in practice if the bolt is hitting the rear of the receiver when running. For myself, the ammo is loaded just above runaway so that the bolt doesn't get a hefty running start at the sear. This is plinking/range ammo tuned to the gun.

 

I started doing the sear test when setting up lowers for use with 22RF. I've got those triggers in the 3-5 lb range. I think the factory trigger is closer to 12 lbs +/-. I'm sure folks will chime in.

 

Good luck, and a good thing parts are still somewhat available. There is at least 1 vendor that has the parts new still.

 

Grasshopper

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. It really doesn't take much wearing or rounding of the sear notches in the bolt

and the sear itself to create a condition where the bolt will jump the sear. Its not

something you can measure, its just recognizing the rounding and battering of the

edges. Both the bolt and the sear are quite hard and more hatm than good would

come from attempting to re-machine the sear cuts in the bolt even though you may

be able to cut it with tungsten carbide. There is no reason to redo the sear since

they are readily available for $15-$25.

When you figure it out let us know what it was...

 

Bob

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  • 2 weeks later...

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