I’ve seen some pretty good arguments about why Bottomless Mags will be one of the best boss fight skill trees in the game, up there with the Red and Green trees for FL4K.
With the right build and gear, you can fire infinitely with some of the most powerful legendaries in the game. You can go the all Vladof route, with 4 Vladof guns and a Vladof shield, with 5 points in Matched Set for 50% increased magazine size and another 35% magazine size from The Iron Bank, for 85% increased magazine size total. The larger your magazine, the more ammo you regen per second from Forge and Redistribution’s combined 10% ammo regen per second. You want to get Cloud of Lead for lower ammo usage and bonus fire damage, Stoke the Embers for increased fire damage (and eventually Experimental Munitions from SoR for even more fire damage), Scorchin’ RPM’s and Click, Click for the greatly increased DPS. Then gear wise make sure you have a fire Vladof sniper, particularly a fire Lyuda, and you will absolutely shred any boss that is weak to fire (any boss with a red health bar that isn’t fire type themselves), and carry a shock and corrosive weapon as well for dealing with enemies that aren’t weak to fire.
Another really strong option is to go the Torgue route instead of the Vladof route. Torgue guns have alternate sticky fire mode in Borderlands 3, and you get a stacking damage bonus for each sticky on an enemy when they detonate. Normally you can’t get that many stickies in each target before your reload, which detonates them, but with BM Moze you’ll have larger magazine size and ammo regen for your current magazine, which will allow you to stack dozens of stickies on a boss, which will blow up for massive stacking bonus damage when you do reload.
BM does lack defensive options, though you can mitigate that problem with the skill that lets you sprint and shoot (enemies have a harder time hitting a sprinting target and you can sprint to cover when your shields are low), by putting leftover points into SoR, and by getting a legendary class mod that gives you 2 points into SoR’s capstone.
I personally thank that a build like this combined with a class mod that gives bonus points to SoR’s Tenacious Defense Capstone will be fantastic for endgame boss fights, especially against bosses that are weak to fire: