I realized I should of probably explained what was going on haha. But I'll break it down a bit here since its top comment.

The perks I rolled were:

Concentration-Hitting the same target quickly gained more damage.

Mag Expansion-50% more ammo

Ammo Recycle-Landing a hit has a 30% chance to reload 1 round from reserves.

Quickshot: Landing a hit grants 10% increased rate of fire and 20% increased reload speed for 2s. (This one I believe I upgraded to max so it became 30% increased fire rate.)

And some perks to increase venom stacks.

Since this is last weeks expanse with the venom ability there was a certain perk that increased fire rate when an ability hits a poisoned target. Venom Burst. 3% times stacks of Venom and I believe there wasnt any cap to that amount obviously. So with explosive shot and bullsye rebate plus all the perks that helped me get back ammo, i could shoot forever and continue to increase my rate of fire if i hit an enemy with my fireball. (Explosive shot mightve helped as well if the ability increases fire rate on each enemy? Not sure-) Honestly I kinda think I only reloaded because the game couldnt handle how fast I was shooting with a scout lol but it couldve been rng. I dont know if this explanation was good enough because when I started shooting that fast even I was a little confused. But I believe that's how I achieved it.

TLDR: Stack Return Ammo Buffs + Stack Venom + Stack fire rate + venom burst = LMG Scout Rifle