Arcane Archers can conjure a stack of 100% returning arrows. Artificers can conjure a stack of 1000 bolts. Mechanics can get a bunch of House D favor and buy more 75% returning bolts when their stock runs dry. That's not AWFUL...but an improvement could be made. I suggest letting mechanics use the trap parts they gather to create ammunition for their crossbows in the field. Perhaps combining mechanical, fire, and magic trap parts could create a stack of fiery bolts that return 50% of the time, or do an exceptional +1 damage, or both.
How about letting mechanics combine these trap parts to create cartridges with 100% returning ammo, or 75%, or something.
How about a red slot weapon upgrade that gives whatever ammo you have loaded a stacking/exceptional chance or returning? You could use trap parts to MAKE an upgrade!
I've put 6 levels of Arti on my mechanic and it was pretty fun. I even had an 18/2 rog/arti split for a while, but I really much prefer a pure rogue, and mechanics are currently the only ranged group that can't make their own ammo. Maybe because mechanics are just a PrE enabled subset of rogues this doesn't look as important...
Anyway. That's my suggestion.