From Brotato Wiki
Silver Bullet grants 25% Bonus damage against Elites and Bosses Secondary Stat, multiplying any damage you deal to Elites or Bosses.
The damage bonus is multiplicative, meaning that if you have an attack that deals 100 damage, and you have 100% Damage, bringing the attack to 200. Then Silver Bullet will multiply that by another 1.25, bringing it to a total of 250 damage. If you had 4 Silver Bullets, the attack would deal 400 damage.