So, several years ago I was looking into this with the topic asking how to set up Arsenal into GunGame, but then things happened midway and I never returned to that, until now.
Well, getting Arsenal to go to lvl 16 is now no problem for me: in Arsenal.py, I just set maxLevel to 16, and increase the range at the comment "# Store all the current weaponset's weapons in a list for easy access." from 0, 8 to 0, 16 (I think this is what I missed back there, as now I properly get to lvls 9-16). To be completely safe, I also set the entry len(self.weaponList) < 7 to 15.
Yet while I get to lvl 9+ properly and also of course set up the Arsenal weaponlist properly, I find that it gives no weapon (just "None"). There still seems to be some limit somewhere that prevents the game from reading entries 8 to 15 in the Arsenal's weaponlist, but I can't find it in any of the files. Anyone knows where I can find this limit so I change it? The only thing I thought could be the issue was __init__.py in the same folder as Arsenal.py which has an entry with as range 0, 8, but I set it to 0, 16 as a test without improvement, so that's not it (I of course changed it back after I was done testing).