Last round bolt hold open

[stingray656667]

Hello,

 

Why would the bolt on an M1A1 not stay back on the last round but instead slam home over an empty mag and onto an empty chamber? When the bolt is pulled back after it has moved forward on an empty chamber it does lock open. Had some range time recently with a friend and his Thompson and I thought it was rather odd that the last round bolt hold open feature didn't seem to work correctly. Is the follower in the mag not moving up enough to trip the sear and catch and hold the bolt open? Could the magazine spring be too weak? The owner of the Thompson wasn't concerned about it but things like this bother me since it should work correctly.

 

stingray656667

[Chief762]

One of my XXX Seymour magazines won't allow the bolt to stay open after the final round but all the others work correctly. I'd suspect the magazines rather than the gun is at fault. Just my $.02

Chief762

[NFA amnesty]

Did you try other magazines?

[stingray656667]

My friend only bought four mags when we went our with the Thompson recently. Two were XX and two were XXX. They all seemed to do the same thing. I didn't notice which manufacturer made the mags but I could ask Andy to see if they were Seymour mags as well. I'll post that information when he gets it to me. Thank you for the replies.

 

stingray656667

[StrangeRanger]

If it locks open when the bolt is pulled back by hand all the bits and pieces are in place and they work but they do so more slowly than the cycling of the bolt in full auto fire requires.

 

Has he cleaned the cosmoline out of the mags? Or any built up of crud from firing them?

[stingray656667]

One of the offending mags was taken apart while we were out and it was found to be clean inside with no firing residue buildup or cosmoline present. I would assume the others were in the same condition but the remaining three were not taken apart and inspected.

 

stingray656667

[reconbob]

The way the bolt hold-open works is that after the last shot is fired

the mag follower pushes up on the front of the trip. The trip pivots - as

the front is pushed up the rear is pushed down against the disconnector.

The disconnector is pivoted out from under the sear lever and the sear and

sear lever snap back up and the sear engages the bolt holding it to the rear.

So anywhere along this chain of events something can go wrong and

the bolt won't stay back. The main culprits are usually a weak magazine

spring and after that the disconnector Spring. If the disconnector spring

is jammed with crud it becomes more difficult for the trip to pivot it. And

what can also happen - and I don't how but it does - is that someone uses

a sear spring thinking it is a disconnector spring. In this case there is no

way the mag Spring would be strong enough to pivot the trip to move

the disconnector.

 

Bob

Edited February 26, 2020 by reconbob