As you know looping is the normal process of going solo, clearing a level, and then because you don't go down at the end of the level you are able to rejoin the level and clear it again.
One small variant on this is to clear a level without picking up any crowns/heat after each kill. Simply leave them on the ground. When you get to the end of the level simply invite your alt to anchor the level, go solo, run back through collecting all the heat/crowns on the ground, rejoin your alt, and repeat.
Items, hearts, and potions are only able to be picked up once, but all Heat, Crowns, and Minerals can be looped and duplicated. Combine this with inviting 3 alts at the beginning of clearing and you can easily make 4-8k/40CE without having to clear the level more than once.
Granted this is a pain to setup, but once you do its SUPER easy money and fast.
Guess you did this gun only? Also, did you manage it in an arena? Can't imagine that being very doable unless you're somehow able to herd all the mobs into one corner then kill them from out of reach
interesting in any case, but sounds far too frustrating to me (mats would be simple as they dont have any magnetism to your character however, and i can see this being worthwhile if you noticed multiples of valuable mats)