Normally I'm a fan of well-made products, but this is a situation where it's an annoyance . The cartridges that came apart without problems were Focal and Triple Print brands. In all cases I am being extremely patient and careful.
I've two instamatics, my Father's Kodak 414 (with the clockwork autowind) and my maternal Grandmother's Minolta Autopak 600-X. They both died about a year ago, so this is a bit of a sentimental project. The Kodak broke while on vacation when I was a chidl, so my parents used my 110 for the remainder of the vacation. I was a kid, so was a bit angry at the time that I didn't get to use MY camera. I need to repair the camera, and think I've figured out what is wrong. The difficult part will be fixing it without breaking it more. A pin (probably plastic) is missing - it holds a spring that presses on a piece of metal that fires the shutter. As far as I can figure, it gives just enough pressure to get the process started. With the pin gone, the spring moves with the metal, preventing it from returning to the cocked position.
I don't remember my Grandmother using her camera, and I've a very good memory.
For my first test I used the Minolta since it works - and incorrectly figured I didn't need batteries. Everything was grossly underexposed (plus it was color film processed as B&W, which made it was worse). After a trip to Radio Shack for some over-priced batteries, the second cartridge came out much better.
A friend at work has a Minolta Autopak 800 that his girlfriend got him from an "antique" store because it was cool. He's never been able to use it, so I'll load a few cartridges for him to try as well.
I plan on having the punch and index pin on a movable piece of metal (or wood or something). That way I can do 110 as well. My best friend would like to use the 110 camera she had as a child. I gave her some of the new Fukkatsu and Lomo 110 for Christmas, but the do-it-yourself route will be more fun.
Curiously, the 110 backing paper (from the cartridges I've disassembled) does not have holes in it. So I tried running a piece of the old Kodachrome in the 126 cartridge with the paper upside down. The film perfs were on the correct side, but with no corresponding holes in the backing paper. It didn't work.