Tuning Umarex M1A1 Thompson Repair

[borne2fly]

The Bolt on the M1A1 is covered in a telescoping plastic. I took some pictures of the upper and lower and moved the bolt, it does not move a whole lot. I made a short video of it. I think it may be a little more difficult to do. What is your opinion?

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YouTube has a video “WE Thompson M1A1 Disassembly & Cleaning, Lubrication Guide”.
This might be slightly different for Umarex but I suspect they both follow a similar design.
It shows the bolt … a substantial alloy piece … which somewhat resembles the bolt in the MP40. If so, it would be a hollow bolt composed of two halves screwed together. And that means you could pack it with lead BBs or pellets before before screwing the two halves back together. The extra weight slows the cycle time quite a bit

 

[ninersix]

YouTube has a video “WE Thompson M1A1 Disassembly & Cleaning, Lubrication Guide”.
This might be slightly different for Umarex but I suspect they both follow a similar design.
It shows the bolt … a substantial alloy piece … which somewhat resembles the bolt in the MP40. If so, it would be a hollow bolt composed of two halves screwed together. And that means you could pack it with lead BBs or pellets before before screwing the two halves back together. The extra weight slows the cycle time quite a bit

Other than the bolt moving back and forth, the Airsoft M1A1 is not the same as the Umarex Co2 BB gun. I am going to have to take apart, at least partially, the Umarex lower to expose the bolt mechanism (very carefully) and hopefully have access to the spring/bolt mechanism and see what is what. It is after all a C02 BB gun and not a Swiss Watch or nuclear reactor. (Taking pictures as I go.)

 

[borne2fly]

Other than the bolt moving back and forth, the Airsoft M1A1 is not the same as the Umarex Co2 BB gun. I am going to have to take apart, at least partially, the Umarex lower to expose the bolt mechanism (very carefully) and hopefully have access to the spring/bolt mechanism and see what is what. It is after all a C02 BB gun and not a Swiss Watch or nuclear reactor. (Taking pictures as I go.)

While you’re in there, see if there’s any way to add a recoil buffer. Anything that might soften that bolt slamming against the receiver.

 

[ninersix]

While you’re in there, see if there’s any way to add a recoil buffer. Anything that might soften that bolt slamming against the receiver.

Yes, I thought about that also. The telescoping black plastic tubes got my attention. So I decided to compress the black tubes which exposed the sear in the gun after I removed one screw holding the tap cover, it slid forward after I pulled the trigger. After looking at it I started to think about where id the return spring? It must be in the larger tube. The Large tube is stationary, so I compressed the small tube and got a good look at the bolt release. There is a small round peg, probably just a stop, I was thinking how about a smaller spring in the small black tube as an additional buffer?
I do not know without thinking about it some more. What do you think?

 

[Sawney Bean]

It’s interesting that you slowed the action on the MP40. I hadn’t noticed that mine seemed unusually fast. The M1A1 on the other hand definitely is. When it’s on auto I’d guess it’s firing at around 1700 rpm.

The firing behavior of it is what makes me like the MP40 more.

I’ve got the Umarex M1 carbine too and while I’d give that top marks for realism, it’s not half the shooter of either of the others.

 

[borne2fly]

Yes, I thought about that also. The telescoping black plastic tubes got my attention. So I decided to compress the black tubes which exposed the sear in the gun after I removed one screw holding the tap cover, it slid forward after I pulled the trigger. After looking at it I started to think about where id the return spring? It must be in the larger tube. The Large tube is stationary, so I compressed the small tube and got a good look at the bolt release. There is a small round peg, probably just a stop, I was thinking how about a smaller spring in the small black tube as an additional buffer?
I do not know without thinking about it some more. What do you think?

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I don’t think adding a spring will do it. I’d fish around to find what stops the bolt’s rearward travel and add a soft plastic stop. The bolt spring should push it forward and the Co2 should push it back, but what limits that rearward travel? Now, it’s possible that a heavier bolt might not travel back far enough to hit the frame. That would be ideal … if the Co2 has enough kick to reload but not enough to ram the bolt against the frame.

 

[ninersix]

I don’t think adding a spring will do it. I’d fish around to find what stops the bolt’s rearward travel and add a soft plastic stop. The bolt spring should push it forward and the Co2 should push it back, but what limits that rearward travel? Now, it’s possible that a heavier bolt might not travel back far enough to hit the frame. That would be ideal … if the Co2 has enough kick to reload but not enough to ram the bolt against the frame.

Well from your description that would be the spring in the larger black tube. I am not a gun smith and that return spring is very strong as can be seen. I needed a vice clamp the hold it back. To access that assembly I would need to remove six (6) screws to gain access to it. Not my first choice.
However I perhaps could slide an absorber either on the front of the small tube against the bolt. I think that would meet you description. Something shaped like a doughnut

See buffer pad replacement

Location where the bolt meets the spring.

 

[borne2fly]

Well from your description that would be the spring in the larger black tube. I am not a gun smith and that return spring is very strong as can be seen. I needed a vice clamp the hold it back. To access that assembly I would need to remove six (6) screws to gain access to it. Not my first choice.
However I perhaps could slide an absorber either on the front of the small tube against the bolt. I think that would meet you description. Something shaped like a doughnut

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See buffer pad replacement
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Location where the bolt meets the spring.

If you attach it to the bolt, then the back-and-forth might knock it loose. I like the idea of a donut shaped buffer. I’d try fixing it to the stationary larger tube (I’m assuming the larger tube doesn’t move).

 

[ninersix]

If you attach it to the bolt, then the back-and-forth might knock it loose. I like the idea of a donut shaped buffer. I’d try fixing it to the stationary larger tube (I’m assuming the larger tube doesn’t move).Yes, the larger tube does move.

Yes, the rear larger tube does not move, the forward smaller tube is the spring return (Pic. 1) see below.
However that brings up a problem. As seen in the picture below the smaller tube goes all of the way in. Thus the doughnut will not work, it will be pushed off the tube. I believe that the protruding post (Pic 2) is the operating devise that pushes on the Co2 (hammer) for releasing the Co2 from the clip.
I am thinking that the only option it the take the enclosed part (removal of all six screws Pic. 3) on the rear silver like housing. I am going to review my exploded parts diagram and have a look before I do it. When I take it apart perhaps I will be able to insert the buffer pad at the back of the spring. We will have to see what it looks like.
borne2fly: I what to thank you for your insights on my little project, they have been very valuable to me in this effort. Thank You.

 

[borne2fly]

Glad to hear I could help .
The donut … I wouldn’t give up on it just yet. If it registers off the smaller tube then then, yes, it could fall out of place if the smaller tube completely retracts. But what if it registers off the bigger tube? To do that, the donut would need to extend back over the larger tube for as far as possible … essentially a sleeve with a step to a smaller diameter at one end.
Just thinking out loud.

 

[ninersix]

Glad to hear I could help .
The donut … I wouldn’t give up on it just yet. If it registers off the smaller tube then then, yes, it could fall out of place if the smaller tube completely retracts. But what if it registers off the bigger tube? To do that, the donut would need to extend back over the larger tube for as far as possible … essentially a sleeve with a step to a smaller diameter at one end.
Just thinking out loud.

Well today I will have some time to look at it (yes take it apart) and have some answer(s). Also; in the future I will need some advise for the "buffer material" and where to acquire it. I know what it looks like, I just have no idea where tom get it from, particularly if it turns out that it will be some complex shape (god I hope not!).
Update: 8:51 AM CST
Oh; I thought I would an exploded parts diagram of the upper receiver of the Umarex M1A1 it identifies the return spring and the bolt. I needed to look at the parts so I did tot screwup something. It is a PDF. Also photo of PDS. Most people do not have the PDF viewer so i made a picture.

View attachment Umarex M1A1 Upper close in diagram.pdf

 

[borne2fly]

MacMaster-Carr sells all sorts of mechanical parts, including raw materials. They’re a bit more pricey than the local hardware store but they have just about everything.
If you’re sure that the thing stopping the bolt’s backward travel is in fact the large tube, then it might be easier (than a donut) to simply slip a soft plastic (nylon, Teflon, etc) sleeve over the large tube, and perhaps 1/16”-1/8” longer. Such a sleeve need not be a precise fit …. a lengthwise cut along the sleeve would allow a slightly undersized sleeve to pry open and clamp down onto the larger tube

 

[ninersix]

MacMaster-Carr sells all sorts of mechanical parts, including raw materials. They’re a bit more pricey than the local hardware store but they have just about everything.
If you’re sure that the thing stopping the bolt’s backward travel is in fact the large tube, then it might be easier (than a donut) to simply slip a soft plastic (nylon, Teflon, etc) sleeve over the large tube, and perhaps 1/16”-1/8” longer. Such a sleeve need not be a precise fit …. a lengthwise cut along the sleeve would allow a slightly undersized sleeve to pry open and clamp down onto the larger tube

Borne2fly; So what I understand you to mean is that the (Teflon or nylon new tube) fits over the large Black tube in the BB gun extended 1/16" 1/8" longer than the black tube Correct?
If I understand this will stop the recoil of the bolt shorter than the normal recoil stroke? Have I got it so far?

So what are the physics behind making the RPM slower?
This is what I am not comprehending in this mechanical motion making the RPM slower?

 

[borne2fly]

Borne2fly; So what I understand you to mean is that the (Teflon or nylon new tube) fits over the large Black tube in the BB gun extended 1/16" 1/8" longer than the black tube Correct?
If I understand this will stop the recoil of the bolt shorter than the normal recoil stroke? Have I got it so far?

So what are the physics behind making the RPM slower?
This is what I am not comprehending in this mechanical motion making the RPM slower?

Yes, that’s correct (your 1st paragraph).

Slowing the rate of fire will come from adding weight to the bolt (lead BBs, pellets, etc). This heavier mass slamming back and forth might hit the stop harder, so I would add a recoil buffer as a precaution to avoid breaking something.