What do you need bolts for after you've puchased all the weapons?

One other use is to fill up your ammo at the shop but that usually doesn't cost too much. Is there any other use for bolts than ammo refills and buying new weapons?

Solution 1:

There isn't much use for bolts beyond buying the weapons. They're more useful once you finish the game, though, for Challenge mode. There, you can buy the Omega versions of the weapons that you have A) found the three card set for and B) You've gotten to level 5.

Purchasing those weapons tends to be a massive drain on your bolt stash; even the Magmabuster's Omega weapon costs 135,000 bolts, with the RYNO Omega costing a cool 1,000,000 bolts.

Do not mistake standard bolts for the golden bolts, which unlock additional features, and have set locations throughout the game.