I would start with a warning: If you print this, your house will be full of rubber bands, in corners and under furniture. I found out that my cat loves to chew on rubber bands and it's definitely not good for her to swallow any. So be cautious for the safety of your pets.

Loading the mag: don't use the very last space for now.

Rubber bands are from a dollar store, no brand. 30 and 40 mm ring diameter.

As for printing:

I printed with 0.2 layer height and wall line count of 3.

Unfortunately supports are needed . Turn on supports for all the parts and don't change orientation. Supports should be enough only from the build plate for easier removal and it should be normal zig zag or something not tree supports.

As for assembly:

Note that the flat side of the gear tooth should look towards the front of the gun.

I hope the video is clear enough otherwise just drop a comment I will try to help you.

There is no afterwork done on the gun you see on the video, it should run smoothly.

As for troubleshooting:

Elephant foot can be a real problem, with mechanical models the good fit is really important. Everything should slide freely if it doesn't file the moving parts a bit.

If you have too strong rubber bands they can squeeze the magazine together causing lots of friction with the trigger system and can cause unreliable operation.