Sticking it to ’em: Impaling vs Piercing Damage

+Peter V. Dell’Orto noted here that sometimes there’s a bit of a mental squaaawwwk! when it comes to comparing piercing vs. impaling damage types.

He makes some good points, and partly, this ties into penetration and injury GURPS-physics.

Impaling damage, by and large, is assumed to result from a deep, often narrow, penetrating injury that gets into your gooey center and punctures important bits.

Piercing damage, by and large, is assumed to result from, er, a deep, often narrow, penetrating injury that gets into your gooey center and punctures important bits.

OK. WTFP? (What are The Factors at Play?)

The difference seems to be that impaling damage assumes a relatively low ratio of energy to momentum; piercing damage seems to be mostly relegated to bullets and stuff that have very little momentum, but penetrate very well.

There are special cases. Bodkin arrows (as defined by GURPS, not hoplologists) change to a (2) armor divisor and pi damage . . . but their energy-to-momentum ratio is no different than the impaling arrows (presumably broadheads . . . and yes I know there’s controversy that the “armor piercing” Type 16 arrows are decidedly not bodkin-points, which seem to be harassment-style flight arrows. Deal with it.).

That being said, here’s a link to an image of some arrow typology. You can see, there’s a lot of them. Some of them (like the Type 9a) are pretty boldly squarish in cross-section, and supposedly make better plate-piercers. Though you still need a wickedly strong bow and a hardened lozenge-shaped arrowhead to even think about it, and accomplishing it requires a few things to go your way, including thinner armor, a properly orthogonal strike, and maybe even poorly heat-treated plate. People will assert that types 7 and 8 are designed to slip through mail, but properly riveted mail is pretty darn strong, and while you may get a narrow puncture, it may also be quite shallow. Ironically, the arrows that have been reportedly confirmed to be hardened through-and-through are the Type 16 compact broadheads (my term). Still, period writings contain censure against smiths who produce improperly soft arrows, so who the hell knows?

Bleargh. Not meant to be a treatise on arrows – but it’s hard to say why GURPS would classify any of the arrows pictured as piercing rather than impaling, though a few are some bastard child of cutting and impaling, it would seem.

But let’s take a “realistic” war arrow: 1400 grains (0.2 lbs., or twice the weight of a standard arrow in GURPS!) fired from a 150-lb bow. GURPS pegs this at about ST 18, or about 1d+4 (or about 2d). Using my rules from The Deadly Spring, it clocks in at 1d+1. In either case, the arrow will have about 160J of energy and about 5.4 kg m/s of momentum. Both arrows are impaling, so they wound like 2d+2 and 2d+8.

Let’s take a .22 LR and a .45ACP, which do 1d+1 and 2d penetration, respectively. With bullet size modifiers, they will wound like about 1d-1 and 3d. The .22LR has only 130J of energy (less than the arrow), the .45ACP has 450J (slightly less than 3x more). The bullets have 0.82 and 3.66 kg m/s of momentum (with the .45 having more), both are less than the arrow.

Wounding? The .22LR is a worse wounder than all others considered here by quite a bit; the .45ACP is either slightly better (by about a point) than the arrow using my “realistic” scale, or quite a bit worse using the GURPS thr-based, more cinematic scale (nearly 4d+1 injury equivalent for a 1d+4 imp arrow). A war arrow can be about the same diameter as the .45ACP, so 3d or more injury isn’t far wrong.


Arrow: 160J  and   5.4 kg m/s momentum. KE/MV = 29.6 m/s
.22 LR 130J  and   0.82 kg m/s                 . KE/MV = 158.5 m/s
.45 ACP  450J and 3.66 kg m/s                .  KE/MV = 123.0 m/s

As it turns out, the .45 ACP has the lowest KE/MV ratio of all the modern bullets I have. Maybe there are some heavy black powder loads with lower KE and higher MV. The upper end seems to be about 500 for the ratio, until you get into saboted projectiles (the M829 tank projectile is nearly 1000!).

But you can see that if “impaling” is for lower velocity ratio stuff, maybe less than 50(?) then there’s really no good reason to distinguish between a GURPS bodkin (armor piercing) point and a regular one. A better division is probably that the AP point is heavier in point and shaft and more expensive with shorter range, the regular point is, well, regular, and you can buy flight arrows with an armor divisor of (0.5) with very lightweight shafts with poor penetration but longer range.

All would do impaling damage. Assuming you have such a thing.

12 thoughts on “Sticking it to ’em: Impaling vs Piercing Damage

  1. I'm wondering, does GURPS give impaling to anything that's neither sharp nor edged?

    Piercing is usually used for narrow, relatively blunt weapons that pierce anyway, due to a small cross section and good velocity. Sling bullets, bodkin points, bullets, beaks, cinematically lethal finger strikes, etc.

    Impaling goes to stuff that's either sharp and edged, or at least just sharp – swords, arrows, estocs, long claws used to thrust, spears, etc.

    That's generally how I decide which gets which when doing weaponry, too.

    The bodkin/broadhead split is interesting in my DF game, because a heavier, shorter-ranged impaling arrow that reduces DR would be the only one worth carrying. Range is almost never an issue, it's damage.
    The old early GURPS solution was reducing DR (-2 DR) and reducing the damage of the arrow (can't recall offhand how much) but leaving it as impaling. That made it a trade-off.

    1. I think the trade off here if you want to invoke realism will need to be cost, with the armor-piercers just being more costly by 2-4x. Because you're right, the range is really not an issue up close. The heavier arrows will be more efficient, which gives them better range – though it will be shortened by the larger mass. The 1/2D range of heavy arrows can be longer than the max. None of this is relevant at the typical dungeon distances. So your "trade-off" has to be cost, and the lowering of damage isn't a bad forcing mechanism, honestly.

    2. Point! Ten bucks a shot is cheap when life is at stake, especially after one or two sorties out.

      Maybe just hit 'em with -1 or -2 per die, so that the -2 is relevant over the entire range of ST?

    3. Perhaps, but then I'd need to re-jigger GCA more to reflect that. That's annoying.

      Really my question is broader – why do you really need impaling separate from piercing? I know from David Pulver's notes why it happened, but does it really make it better to break up "pointy and/or sharp and it penetrates" from "blunt and it penetrates"?

    4. In theory, it could make it easier to do vampires…

      Having bullets do piercing or impaling probably simplifies things greatly for purposes of bleeding rules, and other stuff, because 3e was FULL of special cases where we had to keep saying "Crushing (except for bullets" or treating Bullet as its own damage class.

  2. Honestly, I'd work on nixing them both. Bullets can do crushing like in 3e, arrows probably are cutting weapons, but maybe something like cut-, with special rules for deep injuries. Perhaps. I don't want to overcomplicate (really!) but i always thought that the difference btw cut and crush would be something like "cut bleeds really badly, but heals well; crush doesn't bleed as much, but crippling and disfiguring impairment are more likely."

    1. Note that this sort of stuff would be best done in an edition change, if you do it at all. I'm not attempting to suggest "this is a good house rule for right now." More design-level stuff.

    2. Ah, I see Peter already mentioned this. The huge list of special cases for bullets in 3e were the driving force between classing them as "piercing" in 4e.

      (And, for example, GURPS has a lot of rules like "skeletons take extra damage from crushing" (or whatever) that just don't work if you assume crushing includes piercing.

      Vehicle design has a breakpoint that says piercing transitions to crushing if projectile diameter is 15mm+ and muzzle velocity is sub 300 feet per second.

    3. One thing about arrows as cutting weapons…

      … it lets them kill trees and zombies and hack holes in doors really well!

      An important element of the 4e switch was relating the damage classes to particular types of injury tolerance.

      By making "cutting" distinct from piercing or impaling, it let us set standards where it was harder to shoot up a door or tree or zombie with arrows or bullets than it was to hack them to death or smash them down.

      The damage classes are thus largely related to area of impact.

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