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Thompson Mags Heat Treated? Two-Tone Explanation


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Ive got a batch of 2 tone thompson mags..the bluing is darker in a band across the top. Im wondering if this could be a heat treatment they gave the top 1/2 to harden it? kinda like they did on ww2 1911's

 

what is it?

 

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Edited by huggytree
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I believe what you have are the modified 30 round mags for the M1 Thompson. Cant remember which vendor, Crosby or Sparks-Withington, the mag was to tight for the M1 magazine well so they ground the leading edge of the feed lip for clearance. The discoloration is hot dip oxide to treat the ground edge.
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Right. The mags were originally made of fairly soft steels, evidenced by how easily the feed lips, bodies, and mag bases are bent or damaged. The good news is that the mags are not brittle.

 

The original specs did not mention heat treating. Damaged mags were simply meant to be discarded.

 

I've found a few damaged ones over the years. NOS mags were easy enough to obtain that I whacked the bad ones with a hammer and dropped them into the recycle bin.

Edited by TSMGguy
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Huggytree,

 

1921A nailed it. The one mag I can see is a Crosby which appears to have the front of the feed area ground off. I dont recall seeing the top dipped on all such modified mags, but it makes sense that some could have received this treatment. Some Crosby XX mags also had an exaggerated front lip. The second and third version Crosby XXX mags corrected the problem in manufacture.

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