Silver Bullet is a weak item, despite dealing percentage damage. The reason is simple — low bonus to DPS.
Unlike Tankbuster, this item doesn't bypass resists. That means the taken damage can be decreased. Plus, by default, any solo gun player would kill you much faster, than all of enemy team, with each one having this item.
I want to say, that battles don't take enough time, so item like this could be useful.
Used against non-tanks, your overall damage would be much significant, than potential 6%, even if they have zero resists.
Against tanks, it's useless, because almost in all cases, they will have at least 30% of resist, making your 6% to be 4%.
We shouldn't forget about Plated Armor too.
So, I have two ideas.
First — item makes bullets to deal bonus damage in percentage of current/max health. This passive would work constantly without cooldown. The damage still can be mitigated, but against tanks it would give a decent advantage. The item is useful to non-carry players, but for gun carries, item can perform much better. Perhaps, it would be too much.
Second — make that damage to bypass any resists. Just like Tankbuster does.