Q: Can I use P7 M10 magazines in my P7 M13 so I can hold 13 rounds in a "non-banned" magazine?
A: Now there are a lot of P7 M13 users that use M10 magazines, which only hold 10 .40S&W rounds and are NOT subject to the '94 Assault Weapons Ban, in their pistols, because the M10 magazines can hold 13 rounds of 9mm. While this may work for some, the magazine followers and feed lips ARE DIFFERENT and your mileage may vary. The P7 M10 magazine sell for usually $10-30 less than a P7 M13 magazine, so it may not be worth it to do this.
Q: Are there aftermarket P7 M10 magazines available?
A: No, not to my knowledge.