Have done this time 3 in 1 fidget toy 

combination of 3 toys to get gun shape click spin play with rolls  

to be succesful for this project you will need some magnets 

4x1.5mm x 32 

8x2.5mm x 9

pen spring

due to complex design some parts require printing with supports

when fitting magnets please make sure polarity is attractive to other part where it belong (between rail parts and drum body, between drum and drum body) I recommend to use revers polarity between parts so rail won get attracted to magnet from barrel.

spring you need to cut to length of 7mm and fit it on trigger designated area before sliding it to handle.

Recomended material is PETG as you need to push drum in to holder which can effect with breaking PLA printout as well as triger system.