The Kahr Problems
I just completed an SBR modification of a semi-auto Kahr Thompson. Prior to the conversion, the owner mentioned that his gun jammed repeatedly and did not function well. This presented the perfect opportunity to document the typical issues while working on the gun.
Trigger Frame
Disassembling the trigger group revealed two immediate problems. First, the sear spring was installed in the wrong direction. The spring has a large loop on one end so that it stays captive in the sear spring hole. Compressing this end and inserting it properly takes a bit of effort, but it seems the assemblers either don’t know or are not capable of doing it correctly. As a result, I have observed that 100% of these guns simply have the spring inserted the easy way, with the large loop sticking out.
The issue with this is that the spring won’t properly compress during normal operation. Instead, the large loop end gets squashed when the trigger is pulled, eventually bending and leading to a very hard trigger pull.
The pawl spring was also badly compressed and bent. While I am unsure if it was damaged during installation, I do occasionally encounter this issue. It’s a simple matter to invert the sear spring, which I did, and replace the pawl spring.
Magazine Catch
The magazine catch frequently exhibits a unique problem. There are metal burrs inside the hole where its spring is housed. As a result, the catch tends to push itself out to the left during operation, interfering with its function and preventing it from securely latching onto a magazine. A small file can be used to clean up the hole and resolve this issue.
The Bolt
The firing pin almost always has very rough edges due to the production process. This creates a “gritty” feel when cycling the bolt, and the resulting friction robs energy from the bolt’s forward motion. This can cause the bolt to stop short of being fully chambered, leading to an out-of-battery fire.
Polishing the firing pin’s bearing surfaces greatly improves this issue. While it’s not possible to remove every imperfection without compromising the fit, significant improvement can still be achieved. Additionally, the inner and outer surfaces of the bolt, especially the hammer hole, benefit from thorough polishing.
In this particular gun, the extractor fit fine, but extractor fit will be the subject of another article.
The Barrel
Kahr and most aftermarket 10-inch barrels tend to have tight chambers that are not opened to submachine gun (sub-gun) dimensions. All models of the Thompson benefit from an open sub-gun chamber, which has a slightly larger diameter.
I use a special sub-gun reamer for this task. In the photo, the masking tape marks the depth to which the reamer should go.
Another photo shows how much material is removed when properly cutting the chamber.
Additionally, most Kahr and aftermarket 10-inch barrels benefit from a more rounded chamber mouth. Hand polishing the chamber area afterward yields the best results for optimal function.
Magazines
Kahr magazines produced in recent years tend to be too wide. This results in poor cartridge presentation to the bolt, which is a major source of jamming even when other issues are resolved. If a jam occurs with the round partially fed and the bolt’s face pressed against the side of the case, it’s most commonly a magazine-related issue.
GI magazines, on the other hand, often have squished feed lips. Both wide Kahr magazines and bent feed lips on GI magazines cause similar problems. Improperly fitted GI magazines that sit too low in the mag well also contribute to jams.
You can test a GI magazine by inserting and locking it into place. If it moves up and down a fraction of an inch, it is poorly fitted. A properly fitted magazine should not have any up-and-down movement.
Final Results
Once all these issues were addressed, test firing went perfectly. The semi-auto Thompson functioned flawlessly, proving to be a fun gun for plinking at the range.
~DAN BLOCK