That would be WAY to op. Why not have it just be a set number like 12?
Because I think 12 would make it become obsolete again by the endgame. If you think 12 is not too op to allow it to be crafted with chlorophyte aka post-mech tier, then I don't think my idea would be that op.
At that post-mech point, unless I missed something, the highest damage bullet is Ichor at 13 damage, with cursed as a close second at 12 damage. And the highest damage arrow is cursed at 17 damage, with ichor/chlorophyte as a close second at 16 damage.
So, I was thinking of copy probably only 75% of damage, getting to about 9 damage for bullet and 12 damage for arrow at that post-mech tier. Pretty much a silver bullet and an unholy arrow without piercing.
Then post-plantera will give access to higher damage ammo, 15 damage venom/nano bullet and 19 damage venom arrow. 75% is about 11/14, so the bullet is on par with 11 damage high velocity bullet and the arrow is slightly better than 13 damage holy arrow, but again it only has damage and no special effect so I think it is still at a reasonable level.
Finally, post-moonlord will give luminite bullet at 20 damage, and while luminite arrow has 30 effective damage due to its secondary bolt, I will sadly only count its 15 base damage for this purpose so arrow would have to stick with venom for its damage. But anyway, 75% for bullet is 15, bringing it to venom/nano bullet level of damage.
This is probably the only point that I think it might be too op for bullet, but you can always adjust the number. it doesn't have to be 75% for bullet, you can tune it down to maybe 60%-65% for bullet to still get an improvement over regular musket at post-mech tier at about 8 damage, while post-moonlord will be 12-13 damage, being at cursed/ichor level.
Which kinda contradict my reason in the first place that 12 damage is too low for endgame, but the point is that this can be adjust to what you would consider reasonable.
Or just nerf luminite bullet base damage, it is the only thing messing with my calculation here lol.