Notes on my Merle Bitikofer Thompson Submachine gun .22 conversion unit

[Rabbit57]

Yesterday I tested my Merle Bitikofer .22 conversion unit with my Thompson M1 submachine gun, to establish the rounds per minute speed with each of the springs included in the kit. This is my range report.

My kit came with two recoil springs, one is 9 1/4” long and the other is 11”. I also had a spring that was provided to me by Capt. Monty Mendenhall, who had written an article back in 2010 about a spring that he developed with Merle and Dr. Ed Shaughnessy, a firearms designer and a teacher of advanced engineering at Duke University. Capt. Mendenhall’s spring was 12 1/2”, but he had told me that he was sending me the spring long, so that I could cut it a little at a time, until I got the desired RPM count, and reliability.  He explained that he had been looking at ways to slow the cyclic rate of the gun down to about 650 RPM. I ended up cutting about one inch off of the spring, I might take more, but maybe not. Let me explain why.

I fired a full 30 round Black Dog Magazine full of Federal .22LR ammo, the bulk pack sold at Wal-Mart with the 36 grain hollow point bullet. This was the ammo recommended with the conversion kit, and it worked reliably for me. I clocked the round per minute speed of this ammo through my Thompson M1 submachine gun using a PACT shot timer with a RPM function. The results are as follows-
1. The 9 1/4” spring gave me a speed of 730 RPM.
2. The 11” spring gave me a speed of 913 RPM.

3. After first trying Capt. Mendenhall’s spring full size, I found it was not reliable. I cut the spring from 12 1/2” to 11 5/8’, This gave me a speed of 598 RPM, a nice, steady, noticeably slower speed. Capt. Mendenhall recommended a length of 12", but I ended up cutting a little more. YMMV
I also found that when I used my 50 round drum, the drum converted for me by Dan Block, it was unreliable with the 11” and the 11 5/8” springs. It only worked reliably with the 9 1/4” spring, giving me a speed of 764 RPM. I had an issue with this drum in the past, but now I think the problem has been solved by using the right spring. I will do some more testing, and keep you guys posted on my progress.

Using the Thompson with the .22 conversion unit is a lot of fun. It allows me to shoot a lot of cheap ammo, and gives me plenty of trigger time with my M1. The Mendenhall spring slows the cyclic rate down even more that the slowest spring provided with the Bitikofer kit, it reminds me of a M3 grease gun. The Black Dog magazines are pretty good, not fool proof, but very good. I believe I have sorted out my issue with the drum, but I do think a little more testing is needed, just to be sure.

Edited April 19, 2024 by Rabbit57

[deerslayer]

Im curious about the term "not reliable"  does that mean a magazine feeding jam, or perhaps its strong enough the bolt wont cycle back far enough to eject properly?  What does a failure look like?

[Rabbit57]

Dan, I had two issues with the drum. The first was I would get a round that failed to extract and eject. The empty case was in the chamber, as soon as I cleared the empty case, I could fire again. The second issue was that sometimes the bolt would catch the round about half way, in the middle of the case, and stop. I would safe the gun, drop the mag, clear the bad round and reinsert the mag, and try again. 
Both issues were more of an irritant than anything else. However, when I switched the springs out, I found that I did not have these issues with the 9 1/4" spring. Of course, I only fired two loaded drums, I will keep testing. But I do think that I have solved my issue, the 9 1/4" spring seems to make a difference with my gun.