Semi-auto Thompson "long Recoil" Question

[reconbob]

I have read about a modification of the WH/Kahr semi auto where

the extra material at the back of the bolt pocket is machined away so

the bolt pocket is the same length as the original Thompson. Also longer

recoil spring guides and deeper recoil spring guide holes in the bolt are

needed. The idea is that the gun is not as difficult to cock. This modification

is described in Doug's catalog. (But as we know Doug does not make

finished receivers.)

So - has anyone actually done this? How did it work out? Does it

noticeably reduce the cocking force? (Has anyone measured the

difference?) Does the gun cycle ok?

 

Thanks in advance

 

Bob

[Joe H]

Bob,

 

I looked into doing that when I built my 1928A1 semi auto using your Receiver and a parts kit. Since I wanted to keep the blish lock, the distance from the rear of the bolt to the lock slot and the corresponding cuts in the bolt limits the length of the return spring guide rods and length of slot (for springs & rod) in bolt to a little over 3” (about the same as the NK/Kahr). My return springs bear on new tabs on the actuator and not the bolt. With the bolt closed the length of the guide rods determine the distance to the back of the guide rod assembly since the return springs will jam without the rod support. I built up the space in the rear of the receiver to the rear of the guide rod assembly with a spacer and additional buffer material since the receiver is machined to the original length. Using lighter return springs and the additional buffer material gives a cocking pull equivalent to the original FA TSMG. The system works fine with no issues.

 

The M1 is a different story. The NK/Kahr is essentially a M1 action no matter with version you buy (M1 or 1927A1). The M1 type (NK/Kahr) bolt can be drilled deeper than 3” to allow for longer return rods and ability to use the full length of the Bridgeport bolt cavity. The springs also bear directly on the bolt. Certainly lighter longer springs could be used. This in addition to the longer travel length will probably give a much lighter cocking pull but so does the EZ-Spring modification. Everyone seems satisfied with the EZ Spring. One issue modifying the original GI M1 bolt will be drilling the 3” + holes in the hardened steel. A NK/Kahr bolt should be no problem.

 

I read about this mod on DR's website but I’ve never tried it, as I didn’t feel any need for it for what I was doing. I've never heard of anyone who has. I hope someone else has and could give us a report.

 

Joe

 

[deerslayer]

I've milled a couple out longer when making a post sample machine gun conversion to the semi auto. You also have to mill the actuator slot longer (which then looks a little odd on a 27) but the extra length does slow down the rate of fire on the conversion which otherwise is blindingly fast. I never tried it with an unmolested semi auto bolt to see how it worked maybe I should have....

[Weasel]

Bob, I never did that type of mod for a better cocking feel.

 

On my old WH semi I had cut the two recoil springs to ease up the pull. I kept the rec. untouched. Never had a problem even being aluminum and no buffer.

 

On my 1928a1 semi I cut the recoil spring about in half and had to extend the guide rod by 3/8" to control the spring loading. It runs fine even with the shorter

 

spring and a polyurethane buffer.

 

Weasel