After a week on the struggle bus, I think I'm going to throw in the towel on the Franken 9. I love the concept and there are some really clever design elements. The problem is that I just can't see how to get standard printed Sten mags to work reliably. With a modified mag catch, I was able to get my build to accept a printed DMB Sten mag and feed dummy rounds. But it's hard to get the mag to lock in (because it has to be held right up against the bottom of the slide) and it's very easy to induce a failure just by bumping the mag.
The fundamental problem seems to be that printed Sten mags present the rounds about 0.8mm lower than Glock mags. That means the feed protrusion on the slide just barely catches the rim of cartridge in the magazine. It can work, but there's no margin. Mag fit relative to the slide has to be absolutely perfect. It might work better with surplus magazines (I don't have any to test). It also wouldn't be too hard to remix the printed mags to be a little more compatible, but I'm just not eager to print off a set of "one build only" magazines.
Though I'd really dislike the aesthetics, I think a version of this using Glock mags would be a definite improvement in function/reliability.
My revised mag catch is up on Odysee if anyone wants to give it a try: https://odysee.com/@KopsisEngineering:a/Franken9-Revised-Magazine-Catch:e
