I’m still looking! Does anyone have any info?
you could possibly get 10 or 12 rounds in a single-stack P7M8 mag. Since the taper is relatively low on the P7M13 mag, I'm thinking you might only get 5 or 6 more rounds in a Frankenmag.
For what you would pay in doner mags and welding, you could buy a used Glock or Beretta and some extended mags for what you would pay to get 3 or 4 P7 Frankenmags.
I have seen some made out of other magazine extensions, but they are only good for 2-3 rounds. The M13 mag is a double stack mag that turns into a single stack mag. The follower has tracks that it follows. Lining the tracks up when splicing two mags together is very tricky. I would highly reccomend using the 1991 or later M13 mags as the top mag since they have the extra window. The extra window was put in place to stop the follower from shooting out of the body when dropped. The extra window has an additional stopper to keep the follower in place. When using longer mags, you will need a longer/stiffer spring. The additional tension of the spring could be too much for a pre 1991 M13 mag.
Yeah--- to the OP--- congratulations to both--- members helping members!!!
Here's mine. Holds 21 rounds.