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.