This is legal, because you are not making a "new" magazine. You are merely modifying an existing magazine to work in a different weapon, much as Armalite converts existing M14 mags for use in their AR10s.
Get a 20 round preban 9mm High Power magazine.
Remove the floorplate and internals.
Using a piece of 3/8" OD rubber hose over a dowel, wrap a piece of 100 grit paper around it and re radius the follower.
Don't overdo it, it only takes about four or five strokes.
Using a flat file, remove just enough material on the lips so that a 9mm round will just drop through them.
Gently deburr the lips with some 600 grit paper.
Re assemble.
Voila! A 14 (sometimes 15) round preban .40 High Power magazine!
These work in all of the .40 HPs I have tried them in. They stick out of the bottom a bit, but hey, they hold more ammo.
Note: You can try this with 30 round 9mm mags, but I haven't... yet.
They usually don't work very well in the 9mms, so I haven't bothered to try them in the .40.
Get a 20 round preban 9mm High Power magazine.
Remove the floorplate and internals.
Using a piece of 3/8" OD rubber hose over a dowel, wrap a piece of 100 grit paper around it and re radius the follower.
Don't overdo it, it only takes about four or five strokes.
Using a flat file, remove just enough material on the lips so that a 9mm round will just drop through them.
Gently deburr the lips with some 600 grit paper.
Re assemble.
Voila! A 14 (sometimes 15) round preban .40 High Power magazine!
These work in all of the .40 HPs I have tried them in. They stick out of the bottom a bit, but hey, they hold more ammo.
Note: You can try this with 30 round 9mm mags, but I haven't... yet.
They usually don't work very well in the 9mms, so I haven't bothered to try them in the .40.