a buddy of mine has an old pre ban that has magazine issues, and we were tossing around the idea of using cheap sten mags instead to get it to run right. is this possible without altering the mag housing? i have not tried to mate them up yet but i imagine its not too huge of a dimensional difference.

if it's a pre band open ...don't modify it in any way even to take the sten mags ( They (ATF) would "concider that a new manufactered receiver") if that AP-9 is AP9 a1 (a closed bolt): cut on, brother, cut on, cause other wise it ain't gonna fit.
Many, many Years ago I converted my M11-9s ta stens mags (I HATE Zetel mags!), back then I had a pre ban open_bolt... convereted it to a "striker to firing pin fired -closed bolt-" then later destoyed it; even though it was made before 1982....just not to have any Un-necessary   problems with the Gastoppo..... just 1 old guys thoughts...

yeah i hate zytel mags too, this one i think is metal from what he described, but its the only mag he has for it, and he was telling me the feed lip on the right hand side is damaged so it has feed problems. im sure there is a solution out there, but it has not presented itself.

im also thinking about forming up some new mags based off the style he has. i dont want to go messing with the receiver, its the original and built better than the new ones. to run it with sten mags i might as well make a whole new lower with a mag housing designed for sten mags. im not even going to hack on his property like that, better to leave it alone than have him get pissed at me.

one of the things we have in common is guns and i dont want it to be the reason for bad blood.

I agree about frinendship's....King's used too make some pretty good aftermarket mags for everything out there...but I don't know if they are still in bissness... try making some like ya said or even "re forming" the offening lip.....

To AP-9 user: You might want to look at 32 rd UZI mags. You may have to modify the mag-latch and if the width dimension is off-use spacer(s) in the mag-well.

The AP-9 lower receiver can't take too much before giving up.  CDNN has TEC-9 32 round mags for $17.  It's a better option than modifying a magwell.

Just a little note I have used suomi mags in all 3....tec9, mac 11, ap 9. A little touch up with a dremel and they are ready to go. They used to be had for as little a 7$ but recently saw them at 14$...good luck.

I got a couple of mags from CDNN and I modified them into FUBAR condition trying to get them to fit.  I'll try a Suomi mag and see how it works.  Thanks Grumpy!!!