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What is the difference between 10rd and 13rd uspf internally? Can a 10 be convert to 13? If STI mags can be used for the 10mm convert then can they be used in a reg uspf as .40? What model STI mags? I just bought a uspf 40 and am looking for options with mag capacity.
 

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I'd assume that you can't really convert. See the two little tab things on the sides of the mag that hold the baseplate on? Those are set at different heights for the type of ammo + capacity (I assume, at least...).

So like, the tabs are set higher for 10rd 9mm than 10rd .40. I wanted to switch the baseplates, but yea... don't fit. I assume you'll run into similar problems with different sized mags for the same caliber.
 

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There is no way to convert the 10 rounders to full capacity, unless you're able to TIG weld two of the 10 rounders together. ;)

For higher capacity, check out the jet funnel mags. Hold 16 rounds for .40.

I know there have been some threads in the general handgun section on STI mags that fit the USP's but I couldn't tell you which ones, what it takes to make them work or if they even work at all. ;)
 

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If you had to ask which one to use and how to modify the STI mags to work, then it's beyond your capabilities. It's not worth my time to type out detail instructions describing how to modify them since you won't spend the 1/2 hour to an hour working on them.
 

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What is the difference between 10rd and 13rd uspf internally? Can a 10 be convert to 13? If STI mags can be used for the 10mm convert then can they be used in a reg uspf as .40? What model STI mags? I just bought a uspf 40 and am looking for options with mag capacity.
Yes, they are different both in the spring, base plate, and body. You cannot throw a full cap base plate on there and make it a full cap. The ATFE never would have approved that.

Save yourself some time and money and just get the full cap mags and don't try to cobble something together from your 10 round mags. It costs a lot of money to make a totally worthless mag. I know, I did it. I made several sets of extra-full caps out of neutered mags by welding ten round mags together to make 17 round mags and it is not worth the time or effort. They work, but who needs a mag that sticks out 3 inches from the bottom of the pistol? Not me for darned sure. Making extra full caps out of 10 round mags was an exercise of mental masterbation on what could be done, not on what should be done.
Just go buy new mags. Not in a full cap friendly state? It is just as much against the law to make full caps out of neutered mags as it would be to buy full caps, so you gain nothing legally there either.
 

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Big Bore, what spring did you use in the picture there? I'm looking for something similar to work with a 16-rd mag with a +2 extension.
 

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STI 170 mm spring. Brownells, about $8.
 

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Discussion Starter #8
Thanks for the replies.

I was just trying to see if they was a way to to do it with all of these cheap 10 round mags. i live in a state to get the hi-caps, but thought I would try to save some gun money. As for STI mags, i would spend the time doing, but i will look for the how-to somewhere else.
Thanks.
 
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