pi/pi+ threshold based on projectile mass

[Erling]

As per RAW threshold between piercing and large piercing lies at 10mm. That is, 9x19mm Parabellum is pi, while .40 S&W is pi+.

I have a musing about possible house rule: I'm curious about shifting this threshold from caliber towards projectile mass. One of the reasons for these thoughts was this thread, by the way. Central idea is that 1mm difference between calibers possibly can't produce huge difference in tissue damage. Telling the truth, difference in several grains hardly can do that either, but can provide interesting effects for game purpose.

Currently I consider 155 grain as a threshold. Why 155? First, 155 grain seems to be quite widespread projectile mass for .40 S&W. Second, it roughly equals 10 grams, and round numbers just look nice.

What we get in the world of handguns then. Say, .357 Magnum can push 155 gr bullet with 763 ft x lb, and .40 S&W can push 155 gr with 500 ft x lb. They both are pi+, but .357 has higher basic damage because of higher energy. 9x19mm Parabellum (as far as I know) hasn't loads with 155 gr bullets, therefore it's pi. Another option would be shifting threshold to 147 grains, in this case 9mm Para could be either 2d-1 pi+ (or something about that) or 2d+2 pi depending on cartridge variant. Not very much difference, but I really like .357 Magnum with pi+ more. I think it deserves it: without pi+ it's quite less effective than .40 S&W, regardless of that in real life it's a feared cartridge.

Now rifle rounds. As per RAW 6.8 SPC rifle with 16" barrel does 6d pi damage with Shots 30 and Rcl 2, while 7.62x51mm with 16" barrel does 6d pi damage with Shots 20 and Rcl 3. Both are Bulk -5 or -5*. That is, 6.8 is better in almost every way. But .308 bullets tend to weigh 155 gr or more, while 6.8 SPC bullets are at most 120 gr. If we use house rule and switch .308 damage to pi+, it would slightly compensate for good ol' 308's cons. Again, not that much difference (especially with Body Hits rules from High-Tech), but at least SOCOM operators using SCAR-H in GURPS games can justify it not only with logistics, old habits and expensivenes of buying new 6.8 rifles.

Your opinions invited.

[Toptomcat]

Your players will start trying to make their bullets out of tungsten, depleted uranium, or osmium, depending on budget.

[Erling]

Originally Posted by Toptomcat

Your players will start trying to make their bullets out of tungsten, depleted uranium, or osmium, depending on budget.

Well, there're always some ways to "abuse" the system, or, rather, take advantage from certain breakpoints. Even without the house rule switching from 9x19 Para to .40 S&W gives huge advantage without almost any drawbacks. Also rounds like APDSDU and tungsten-core APHC are already there, and the question is if player characters can put their hand on 'em. And finally, my players don't tend to exploit system in silly ways.

And that goes without saying that changing bullet for a heavier one without further elaboration will lower muzzle energy and thus lower the penetration.

[Fred Brackin]

Originally Posted by Erling

Currently I consider 155 grain as a threshold. .

Historically that's not a strong choice. Back in the 60s and earlier the great majority of US cops carried .38 Specials loaded with 158 gr bullets but these were definitely P and not P+.

It can get worse. The British military pistol load known as the ".380/200" used a 200 gr bullet that only had a muzzle velocity of 625 fps. HT rates this as 2D-2. A downgrade from the 2D-1 of the .38 S&W that the .380/200 was based on. <shrug> Balistics theories of a pre-scientific era were sometimes weird and don't hold up very well these days.

You're not going to be able to use mass without reference to velocity and get sensible results. Velocity definitely figures into base damage calculations and also forms the dividing line for which rounds below 8mm are P and which are P-.

With your choice of rounds you have been considering you have an "implied" velocity floor but you're going to need to use explicit considerations of velocity in a full system.

[Erling]

Originally Posted by Fred Brackin

Historically that's not a strong choice. Back in the 60s and earlier the great majority of US cops carried .38 Specials loaded with 158 gr bullets but these were definitely P and not P+.

It can get worse. The British military pistol load known as the ".380/200" used a 200 gr bullet that only had a muzzle velocity of 625 fps. HT rates this as 2D-2. A downgrade from the 2D-1 of the .38 S&W that the .380/200 was based on. <shrug> Balistics theories of a pre-scientific era were sometimes weird and don't hold up very well these days.

You're not going to be able to use mass without reference to velocity and get sensible results. Velocity definitely figures into base damage calculations and also forms the dividing line for which rounds below 8mm are P and which are P-.

With your choice of rounds you have been considering you have an "implied" velocity floor but you're going to need to use explicit considerations of velocity in a full system.

Alright. I'd pick 1125 ft/sec as a threshold then. It's a common muzzle velocity for pi+ 155 gr rounds like .40 S&W and .45 ACP. There do exist modern .38 Special rounds with 158 gr bullets pushed forward at ~1150 ft/sec, but I don't mind giving them pi+. Besides, they're definitely +P (extra powerful).

[DouglasCole]

Originally Posted by Erling

And that goes without saying that changing bullet for a heavier one without further elaboration will lower muzzle energy and thus lower the penetration.

This doesn't go without saying - if you don't change the powder charge then you'll probably see a pressure spike as the higher inertia of the bullet keeps it in place a tetch longer, and that may well increase the overall force x distance equation.

that can lead to "ka-BLAM" incidents in pistols with unsupported chambers (this was a problem with Glocks in .40 with heavy bullets for a while). Tuning the powder charge down a bit to keep the peak pressure within spec will thus maintain roughly the same force-distance, and thus muzzle energy, curve.

You can, of course, deliberately tune it down to lower energy, but there's nothing a priori saying it must be so. The velocity will drop given equal energy, but the muzzle energy, and thus penetration, could easily stay constant or even increase if tuned to do so.

[Varyon]

Originally Posted by Erling

Central idea is that 1mm difference between calibers possibly can't produce huge difference in tissue damage.

This is simply a resolution issue. If GURPS were run by a computer rather than a human being, an option would be to actually have a sliding scale of wounding modifiers, so instead of Pi vs Pi+, you'd have a range between x1 WM and x1.5 WM.

That's not really an option at the table, so GURPS simplifies things by having some bullets be Pi with a WM of x1, and slightly larger bullets be Pi+.


As for basing things off weight, first off this is still going to cause the same breakpoints as earlier, but now there's no longer any real basis in reality for the WM's. The WM's of piercing weapons are based on the width of the permanent wounding channel, and that's much more a function of projectile width than projectile weight (weight can sort of come into play with yawing projectiles, but that's not actually weight but rather length). The weight has other effects, of course - heavier bullets lose energy more slowly, so have relatively longer 1/2D ranges, and it also plays a role when it comes to penetration. Related to penetration, I believe weight can also influence the temporary cavity produced by a round, but even if you feel the temporary cavity should have an effect (GURPS default*, as explained in Tactical Shooting, is that it has no effect), WM isn't really the way to go (also I think velocity is more important than mass for temporary cavity size).

*Probably the easiest way to bring in temporary cavity effects is to just follow Basic Set and not have blowthrough be a thing, so arguably GURPS default accidentally has temporary cavity having an effect (the real reason is simplicity, I believe). For the more extreme temporary cavity myths, when a target is shot for a Major Wound, do a random hit location roll - wherever your roll, that hit location takes equal penetrating damage, with no DR, as the original wound. Yes, this means a character's heart (Vitals) can be damaged by a shot to the arm, or his brain (Skull) be damaged by a shot to the leg.

[Polydamas]

Any system which divides a continuous reality into a few categories for simplicity creates problems at the boundaries. This is the Continuum Fallacy or the Sorites paradox and its the same whether you break up the continuum of bullet diameters or the continuum of bullet weights.

I like Doug Cole's model balancing energy and projectile diameter because the trade off between wide weapons, which can make the target bleed out quicker but are more likely to get caught on fur/clothing/bones/armour, and narrow weapons, which pierce deeply but may not do enough damage to kill quickly, is probably older than language. Its certainly an issue which almost anyone can understand, whereas only nerds care about the weight of bullets in grains.

[Erling]

Originally Posted by Varyon

This is simply a resolution issue. If GURPS were run by a computer rather than a human being, an option would be to actually have a sliding scale of wounding modifiers, so instead of Pi vs Pi+, you'd have a range between x1 WM and x1.5 WM.

That's not really an option at the table, so GURPS simplifies things by having some bullets be Pi with a WM of x1, and slightly larger bullets be Pi+.


As for basing things off weight, first off this is still going to cause the same breakpoints as earlier...

Sure, I don't dispute that. I'm aware of resolution issues and don't claim that RAW is bad or wrong. I just find it that changing threshold just a little bit can make sense with regard to examples I mentioned in my first post.

Originally Posted by Polydamas

wide weapons, which can make the target bleed out quicker but are more likely to get caught on fur/clothing/bones/armour, and narrow weapons, which pierce deeply but may not do enough damage to kill quickly, is probably older than language. Its certainly an issue which almost anyone can understand, whereas only nerds care about the weight of bullets in grains.

Trick is that in GURPS pi+ ammo for firearms is better in both killing and piercing, since even modern APHC ammo turns pi+ into mere pi, while pi gunshots become pi-.

[Ulzgoroth]

Originally Posted by Erling

Trick is that in GURPS pi+ ammo for firearms is better in both killing and piercing, since even modern APHC ammo turns pi+ into mere pi, while pi gunshots become pi-.

That doesn't make it better for piercing, that makes it better for killing after piercing.

Base damage is what makes a round (before applying bullet types) better or worse at piercing.