I finally got around to cutting down one of two 35-round PT magazines. I used the same process you've seen here before: cut out the mid-section and JB Weld the bottom section to the top section. Sand the JB Weld smooth and spray on a little tactical black. I ended up with a magazine about 5" long that holds 13 rounds.
M&P1522 pt mags 001.jpg
M&P1522 pt mags 004.jpg
I also went one step further to increase the capacity by taking about 1/2" off the bottom of the follower. This probably got me 2 extra rounds that I wouldn't have originally had.
M&P1522 pt mags 007.jpg
After reassembling the magazine I had some issues dry feeding all the rounds from the magazine. Careful measurement and comparison to the S&W mags led me to this fix: the lefthand feed lip was coming too close to the ejector when loaded into the rifle. The ejector pressed down on the round, causing the bolt to skip over the rim and not feed rounds consistently. I first took a little material off the top side of the lefthand feed lip. Then shaved just a little material off the top edge of the mag lock slot, these things in conjunction allowed the feed lip to sit just below the ejector like it is suppose to. I also clipped the edge of the lefthand feed lip like the S&W magazine. Hopefully you can see this in the picture compared to an unmodified PT mag on the right.
M&P1522 pt mags 012.jpg
When loaded, the rounds pitched tip-up at a very sharp angle. To bring the tip down a bit, I took the follower and reshaped the angle at the top of the follower a bit. You can see this comparison in one of the first pics.
Today I shot close to 200 rounds through this magazine with no problems at all. I love this magazine and can't wait to modify my second mag so that I have two.
M&P1522 pt mags 018.jpg

Anyone interested in more info on how to cut down your magazines or even tips on how to make your PT mag function properly, send me a PM. I can't say I'll make your PT run perfectly, but so far all the tweaks that I've made have improved the function of this magazine greatly.

I like it.

I'll send ya three of em.

You can mod them and then keep me for your effort.

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Molon Labe!

Don't sell yourself short, you're worth more than 3 mags.

I am curious to see how these last in the long term under loaded pressure.

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Save time, see it my way. LP

great work! looks like it could be a production mag.

HAHAHA!

Damn auto correct!

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Molon Labe!

I have an idea that JB Weld will outlast the magazine, but we'll see.

Correct. A few pounds of spring pressure on the floorplate is not going to make JB Weld fail.