AR15 style spring loaded magazine with 5 bullets.
Put a drop of super glue into the magazine before putting the spring in.
Add bullets to hold spring tight to the bottom of the magazine while glue dries.
The spring is fully printed out of plastic and will have memory upon the tension after some time. I suggest leaving bullets out most of the time unless you don't mind the compressed spring. Bullets will still hold tight into the magazine and they will not fall out.