I play with a group where the DMing duties get passed around every now and then, but no matter who has the job they always face the problem of our one player who almost always rolls crits, no matter what d20 he uses.
What ends up happening is that in order to create an encounter that he doesn't blow through a monster in one hit every round (doesn't matter if it's level 1 or 15+) the monsters have to be beefed up to the point where they can easily kill everyone else in the group. Boss fights have ended in three rounds or less. Everyone else in the group feels pretty much useless most of the time, like we're just there to keep things busy until he can vaporize them. There's generally not much challenge, so not much drama.
He's played a dwarf ranger, and the two weapon fighting style with a dwarven waraxe only made it worse. He played a half-orc inquisitor and I had to change the racial proficiency with greataxes and falchions for the sake of balance, convinced him to go ranged but heads still exploded. Now he's playing a dwarf gunslinger and oh-dear-gods-the-musket-does-how-much?
Once we gave him a cursed weapon where every crit he rolled would be a fumble. He had no way of knowing it was cursed. Only two people knew what the weapon did, me and the DM. The topic was never discussed while he was in the same building, yet when he equipped the "badass" new axe, it was his worst night of rolls ever, never came close to rolling a crit. The force is strong with this one.
We've watched him. He rattles the die around in his hand, he tosses it into the air, it rolls, and 9 times out of 10 it's a 19 or 20. And if it's not that, it's not too far off the mark. We've made him use a different die. We've made him use our own d20s that have never given up a 20 to save our lives. Same result. We made him use a dice rolling app I have on my ipod and that brought his rolls back down to a normal level. He still hit, but he wasn't doing 4x damage. It made our DM happy, but our high roller wasn't pleased with not being able to use real dice.
Any tips on how to deal with a player with unreal dice rolling luck that will keep him and any DM happy?