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.