Not the length, it is the taper of the case.....and corresponding taper of mags

doubletap

My experience in trying to get it to work in an Uzi was that the mags are the wrong width to work with the .40.  It does ok as long as you don’t go over 10-12 rounds.

Since the dimensions of the Colt mag are pretty close to the Uzi mag, I would assume you would have the same issues.

The mag is designed for an optimal width for a double column of 9mm cartridges. When the Uzi .45 conversion was dreamed up, the mag width worked, as you get a single column of the fatter cartridges.

The problem arises with an in-between diameter cartridge, resulting in a non-optimal double column. The cartridges are only nested together half way. As you continue pushing more cartridges down on the column, they want to nest more closely together, causing them to push outward. It gets to the point that they jam up in the middle of the mag and won’t roll up or down any further.

The picture should give you an idea of what I mean. The .780” stack width works great. Narrower stack, not so much.



I tried this using Uzi IMI .41AE and .45 ACP mags, Promag .45 mags, and I even tried a Rock River .40 mag. All had this issue.

The closest I got using any of these was to install a set of ribs inside the mag body so the .40s would stack in a single column. This of course greatly reduces the mag capacity.

For the Uzi, I was able to go to a Grease Gun grip mag housing and use GG mags. I can reliably run 35 rounds of .40 in a GG mag. Unfortunately, there aren’t a lot of AR15 lowers designed to use GG mags.

Excellent visual example and info. Thank You.