Endless Musket Pouch

It does not automatically become your ammo. The game checks the dedicated ammo slots first and then walks your inventory from the top left, firing whatever it finds first. So the pouch is a default or a fallback depending purely on where it sits. The usual trick is to keep it below a stack of specialty rounds: you burn the good ammo first, then drop onto infinite Musket Balls without touching a key.

Any fight measured in thousands of shots. The Dungeon Guardian is the obvious one, since every hit gets chipped down to a single point and you are not going to carry enough bullets by hand. Long statue farms, event waves and AFK setups are the other half of the answer. For a normal boss fight, better ammo beats endless ammo every time, because the pouch is stuck at the bottom of the bullet ladder.

Research already gives you a bottomless supply of anything you have unlocked, so building this is work for nothing there.