Converting STI mags for the USP?

[Garrett]

I keep toying with the idea of using a USP for USPSA. I've got a number of 16-round mags and a jet funnel. However, that is still somewhat limiting, when I've got other guns that hold 20 rounds in a 140 mm mag for that sport. Some have said they had shoehorned various aftermarket Glock basepads on the HK mags with some success.

I'd read here previously that Bruce Gray had been converting STI mags to work in the USP. I tried one of my STI .38 Super mags, and it fit just fine. If I wanted to convert a mag, would it be as simple as taking an STI .40 mag and cutting a catch in the appropriate place? Other than typical mag tuning, is there anything specific to converting one of these to work in a USP?

[racine]

I keep toying with the idea of using a USP for USPSA. I've got a number of 16-round mags and a jet funnel. However, that is still somewhat limiting, when I've got other guns that hold 20 rounds in a 140 mm mag for that sport. Some have said they had shoehorned various aftermarket Glock basepads on the HK mags with some success.

I'd read here previously that Bruce Gray had been converting STI mags to work in the USP. I tried one of my STI .38 Super mags, and it fit just fine. If I wanted to convert a mag, would it be as simple as taking an STI .40 mag and cutting a catch in the appropriate place? Other than typical mag tuning, is there anything specific to converting one of these to work in a USP?

Your best bet would be to ask Bruce Gray directly. I remember someone doing that out on the west coast but I also recalled that reliability might have been an issue. I'd consider using original mags with extended base pads to get you to 20. The limiting factor for those hicaps are the plastic bodies that stretch and may not drop free when needed or perhaps be fragile. For as much as they cost, HK should have used Glocks metal liner to get durability.

[Garrett]

I bought an STI mag from Brownells, and it showed up today. Like the .38 Super mags I tried, the mag body slides up in the magwell and has a decent fit. It doesn't act like it's going to want to drop free though. So that may be something I need to work on. But it doesn't feel like it's binding hard either, so it should be doable.

Now I just need to cut a square hole in the front of the mag in the right location and I should at least be able to try it out.

[PINMAN44]

Garrett,

I wouldn't waste your money or time. Load 16+1 and do a reload or two

I'd rather sacrifice a little time for ultimate reliability. In the end you'll be saving time by having a gun that continues to go bang.

Rob

[Garrett]

Point taken. 16 reliable rounds is good. 18 or 19 reliable rounds is better.

I wasn't going to try running them In a match until I had all the bugs worked out.

[PINMAN44]

Then I'd stick to your STI.

[RAZZ]

Bruce Gray used to do the mod, but when I called a while back to inquire about it, they pretty much said that he no longer works on such menial tasks. It seems like it would be an easy mod, but it needs to be done right. It's not just a notch cut in the front of the mag body. It has to be cut (or material added) and then pulled outwards at a specific angle in relation to the mag release. If the notch is cut too deep, or the hole is too big, the mag release will bind, or extend all the way into the magazine itself, creating all kinds of problems.

I know Larry Houck used them while he was shooting an Expert for HK years ago. He's on this board, so you might shoot him a PM for more info. A .40 Expert with 20+1 capacity would make a great Limited gun.