Maybe he has to "roll" for a success to re-roll the gun dice. A 10% chance of success at the start and then it goes up to 20% on the next time he tries if the first one fails. Also if it fails, the gun jams until he can reroll again so he can only switch it when he's not in the middle of battle or it'd be a big risk (would be cute then to use his future prediction power combo-wombo).

Timelimit in hand regardless, 30 mins-an hr is prob ok and then an hr-two hr cooldown period!! Once it fades, maybe he can restart the dice.

SOMETHING LIKE THAT. I'm bad with visualizing ic time in rp sometimes so the timelimits aren't hard, just cooldown periods + rng to switch might be good to me to keep all of them as is.