M*CARBO Brotherhood

Running .40 in the 10 mm

I have read that I can run .40 cal in my 10 mm clip. Is this a smart/safe practice?


I have no idea if this is an ok practice, but I have a 20 rd Redball .45 cal converted to 10 mm (10 round mags feel like having small feet) and she shoots it really well; considering I only used needle nose pliers to bend some metal downward very slightly so the 10 was a bit more snug in the magazine.

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@AStartledGuy The .40 and the 10 mm both headspace on the front of the brass. You can do it in 10 mm revolvers with a moon clip used on the .40. You might get away with it in a semi-auto pistol but most likely the firing pin will push the .40 further into the chamber instead of firing it. You have a blowback operated gun, not a locked breach Glock whose extractor grabs the ammo’s rim while chambering it. I believe @Dred has one. Maybe he can answer your question better than me.


Yes, you can fire .40. Note that you will be headspacing on the extractor rather than on the mouth of the casing. So, your extractor must be functioning properrly and repeated firing may lead to extractor wear or failure.

I run a bit of .40 through all my 10s (both). This confirms viability of the firearm as a platform to fire .40 if it ever gets that bad. 40 has run fine in all 4 of the 10s I’ve tried.

It is a good practice to clean the chamber after firing 40. The chamber rim 10mm headspaces on has been exposed to blast from the 40. Clean it to ensure 10 finds battery properly.

On the magazine front - they can run. I’m careful to load them flush with the back wall of the mag. I too can run 10mm from 45ACP Redballs but it just doesn’t feed smooth enough for warm and fuzzies yet.