Best setup, based on over 700 levels of play (before the game crashed from an out of memory error):
Quad barrel
Shotgun ammo
Canister mag
Sticky trigger
Bouncy grip
Backwards-shooting stock
You can swap the Bouncy and Sticky properties on the trigger and grip for the same effect. Mini gat barrel instead of quad will give you an endless stream of bees, but fewer projectiles/shot.
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