I have picked up a few weapons that have alternate modes. Either alternate element or alternate zoom level. I can’t figure out how to change between them. So I just use them in the default mode. I’ve looked online and have found people saying press Y , Triangle or D-pad but I’m on PC not a console.
Maybe go to Settings->Controls and check what key is assigned to this.
Should be the same key as what swaps elements with Maliwan gear, between Sticky and Impact for Torgue gear, between weapon modes for Vladof.
If you haven’t got this button dialed in, you’re in for a treat - that’s like half the weaponry right there.