From Brotato Wiki
Pumpkin
Item
Tier 2
Item
Pumpkin is an in-game item that can be bought from a Shop or can be found in a Crate. Most Items modify the Stats of the player, and some of them provide additional effects.
Effect
Some Ranged Weapons have Piercing, allowing their projectiles to pierce through, and hit more than one target. However, weapons have a default 50% damage falloff on the projectile for each time it pierces. Each Pumpkin you have reduces this falloff by 15%. So if you have a pumpkin, only 35% damage is lost on each pierce.
- Pumpkins can be stacked, removing 15% each. If you have 4 pumpkins, for a total of 60% reduced damage, the 50% damage falloff is completely removed, and the attacks will deal full damage on each piercing/hit.
- Some some weapons have innate piercing, and come with a different damage falloff.
Double Barrel Shotgun only loses 30% Damage per Pierce, so it only needs 2 Pumpkins for full damage. 
Shredder has a default damage falloff of 0%, meaning it won't need any Pumpkins - The Damage falloff modification done by Pumpkin is affected by the All Damage Modification changes that characters such as
Entrepreneur, who only gains 7.5% Less Damage falloff from a pumpkin (Due to 50% less damage modifications), or 
One Armed, who gains a whole 30% Damage falloff per Pumpkin. (+100% damage modifications).
- See Piercing for more info and examples.