Ammo mod: Armour Piercing[PEACH]

[Rabidmuskrat]

Why must all attempts at piercing really heavy armour rely on hitting weakspots? Why can't we just hit the toughest point with enough force to go right through?

Ranged Ammunition Modification: Armour Piercing Tip
Any ranged ammunition that does piercing damage can benefit from this modification. It increases the base cost of the ammunition by 5 times and reduces the damage done by one size category. It stacks with all magical enchantments and all other modifications.
Ammunition with the AP Tip modification reduce the effective AC of their target on attack for the purposes of determining whether the attack hits by reducing the damage dealt.
Any attack that misses due to Armour or Natural Armour (not shield) bonuses to AC, instead hit, but suffer a -1 (untyped) penalty to physical damage for every point the attack roll misses by. This penalty can reduce the physical damage to 0 or less, in which case the attack misses.
If the attack would be a critical hit, it suffers no penalties in damage unless the roll that threatened the critical hit is insufficient to hit the target normally due to the previously mentioned sources of AC, in which case penalties apply due to that roll (not the confirming roll). It does not aid in confirming a critical in any way.

For example:
A medium longbow does 1d8 damage. A medium longbow firing AP Ammo would do 1d6 damage instead (as a small longbow). Now suppose you attack a guy in full platemail with the bow. He has AC 10 + 8 (Full plate) = 18. Your attack roll is only 16 and if you were firing a normal arrow you would miss. However, an AP arrow would hit for -2 damage, so on a roll of 4 (from a d6) the arrow would deal 2 damage.
If your attack roll was instead 10 and your damage roll 6, the arrow would 'hit' with a -8 penalty to damage. This would reduce damage to -2 and the attack would therefore miss.

The goal behind this is to make it possible for larger groups of low level characters to still deal damage to higher level/power characters and monsters. It is also to bring ballistae and such into the realm of the useful, since they gain such precious few bonuses to attack.

OP? Underpowered? Something similar already existing?

[TuggyNE]

Well, to start with, call it a bodkin point, because that's what that is.

The critical hit section needs to be reworded a bit; try "If the attack threatens a critical hit, it suffers no penalties in damage unless the roll that threatened the critical hit is insufficient to hit the target normally due to the previously mentioned sources of AC, in which case penalties apply due to that roll only (not the confirming roll). This modification does not aid in confirming a critical in any way." You might also write up an example for that, mentioning that a critical threat that barely hits will have the damage penalty multiplied along with the damage done if it confirms.

[Xerlith]

Is this mod due to magic or just superior quality?
If the latter (and it seems so), you may restrict it to only be able to be applied to a masterwork weapon and maybe in exchange make it so the ammunition damage penalty works also the second way and if the attack roll exceeds the AC, it deals additional damage equal to the difference between the roll and the AC.
Of course, it's not really "armour piercing" anymore then.

[Rainbownaga]

The main downside of this is the pain at having to calculate the damage reduction, particularly at higher levels when a character can fire off 5 arrows in a round with practically no optimization, and not necessarily at targets with the same armor class.

More importantly, since your goal is to make large numbers of enemies more threatening, the already tedious task of rolling for a large group of npc's becomes even more of a pain: 10 npcs with rapidshot could fire 20 arrows, and rather than just saying hit/miss you would have to calculate damage on attacks that will quite likely be 0's and 1's against high level characters.

And you can't just roll damage together because each attack penalizes its own roll with a floor effect.

Why not drop the damage by two categories and make it a strait touch attack on the first increment? That way you can roll all damage rolls together and save a bit of maths.

Or you could reduce the weapon damage by natural+manufactured AC/5 so that the amount you are subtracting could at least be a constant for each target.