Yes, I even tried shells with larger skirts to hold the shell in place, but I guess the fact it breaks the tightness of the breach seal reduces the power output by a lot, so allowing the entire cartridge to be propelled forwards with its content has got me better and more consistent results, and despite being made of PLA, the shells pack quite a punch when they don't spin.

It's just for fun, so I'm trying not to have too many rules around it, and if anyone has any ideas, just let me know and I can give it go.

On the chrony thing, I don't really have one. I think right now the numbers will be quite inconsistent though, but happy to try. Maybe you can even have some shells to give it a go at your end too. I wanted to test it with a .22 too, but all my 22s have mags on and I didn't really wanted to take them out to test it.

Well, I've kinda designed and 3D printed .177 shells to be used on a rifled barrel instead... So I designed, printed, and filled it with  no. 9 shots - between 7 to 12 pellets per shell - and to my utmost surprise, it worked!! Definitely not hunting material, but it never jammed, and it's so freaking fun.

After a few tests I figured that allowing the shell to be propelled forwards help a lot on neutralising the rifling, as the shots seem to spread only after - or just before - leaving the barrel, and it adds an extra punch and kinetic energy when it connects to the target. I tested it with a youth 6FPE and after a few different designs, the one I found worked the best was the one below:

The cartridge spins some times, but when I tried a more aerodynamic model I didn't really get too good patterning:

Still working my way through it, but it has been so fun that I decided to share it here. Maybe once I get a more final design I can send some to some forum members to test it out.