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Are any of these reamers to Thompson military specs? Which one?




Are the headspace gauges marked, government, the ones used for military Thompsons?



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My experience with TSMG chambers is that they are slightly larger than the standard .45acp pistol chamber, especially "match grade" pistol chambers. Fired TSMG cases sometimes look slightly barrel shaped but this is due to oversize chamber. Some of the WH made barrels had chambers reamed with standard pistol reamer and were too tight causing malfunctions. I had to open up the chambers on some of the M1 thompsons I purchased from them years ago. I believe some of the late made 1928s had similar problems. You could mic an original barrel and compare it to a Numerich/WH barrel and see the difference.


This is a good question for PK. You can bet he has the correct reamer.


Greg Fox

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Thanks Greg, So you think those sold at Brownells are for match grade types only. Well, does anone out there have any headspace gauges for sale, suitable for the GI Thompson? Please let me know.
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Many of the NAC barrels were/ are undersize for even standard pistol specs.


I know of no one who makes a mil spec reamer off the shelf; they have to be special order and the size specified.


Standard 45 acp head space gage can be used to check headspace on a TSMG, but the process is not as “normal”; you need to understand the relationship of the parts and their tolerances to make the determination.


I can not recommend the use of a chambering reamer to “clean” a chamber, especially with the barrel in place. Unless you have a great deal of experience, it is extremely easy to cut the shoulder and increase the headspace, requiring the barrel be set back to restore same. Great caution must be exercised here.


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Thanks, PK.

After checking around, I soon realized that obtaining tools and information for Thompson headspace is pretty esoteric with ordinary gunshops and smiths. I think I'll try a recoil spring and different ammo, as Murray and others suggested. If that doesn't make it, then I'll send it off.

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