Doing a custom body rework of one of my airsoft guns, which does not have a threaded barrel by default. This kit has a barrel thread adapter and a silencer which is exactly designed for my specific application. It is probably not of much use to a lot of people as it is, but the principle might give you an inspiration.
I have printed only the adapter and internals, as I am using a mylar screw isolating sleeve M30 as shell, and glued everything with epoxy, gave a very strong part.
In case you do not have a suitable sleeve I have included a setup called "complete" with a tube as shell. And just because I am such a nice guy files are both STL and STEP included, let me know if you need any other for edits.
I have not idea how well the "complete" model will print, I did the internals only and placed them next to each other, with brims for extra adhesion (I like those).
Final Assembly (if not printed "complete")
First I wrapped foam around the grids, so you have some sort of silencer fuction. Recommend using black or dark gray plastic foam with non-closed surface. If held it in place with standard tape.
Apply 2-k epoxy glue or similar around the parts which shall be fixed to the inside of your shell, be generous at the grids' front ends. Push together into the shell.
I did a paint job by first preparing the outside with 120 grit sandpaper, if you use mylar this is 100 % necessary. After three laysers I got it perfect.
Regarding the barrel adapter: I pushed it onto the barrel and drilled two M4 threads into it. The holes on each side allow perfectly for M4 countsunk head screws.