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Old 12-11-2020, 06:09 PM   #1
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Default Base gun damage on calibre and gun size; armor divisor on kinetic energy?

In Pyramid 3/44: Alternate GURPS II: Survivable Guns, David L. Pulver describes an optional rule for rifle-class weapons, and lays out the reasoning for it.

In short, the damage of guns in GURPS is scaled by their performance versus hard armour. Humans, on the other hand, are not hard armour; and the argument is made to scale the damage of rifle-class weapons down, and their armour divisor up. This gets a comparable result versus armour, while having a perhaps more reasonable effect on the character, who has not had that much larger a hole put in them.

In this, I suggest going a step further. While the same bullet going twice as fast, will deal more damage from hydro-static shock; it is very likely going faster (or slower) because it is shot out of a longer (or shorter) barrel. It seems, then, that basing gun damage on the size of the bullet, with a small adjustment for how fast it is going (or the length of the barrel); and applying the kinetic energy in general as an armour divisor, could give useful results.

From here, I don't actually have the ballistics knowledge to go further. To show that I'm not going to sit here and ask others to do all the work, I'll take a shot at it, anyway. ;)

For a "rough draft" on estimating damage, I would divide a gun's calibre of width by a comparison calibre, such as the Elgin Cutlass Pistol, .54 caplock, for its mathematical niceness as a 2d pi+ weapon with an implicit armour divisor of (1). Half of this decrease or increase would then be a change in damage, the other half would be a change in pi multiplier.

For a Nagant R-1895, 7.62x39mmR, that would round to 1d+2 damage, and pi (no multiplier). The bullet looks rifle-class, but is shot out of a pistol, so an AD of (1.5) seems appropriate.

For a sanity check of damage done, the Glock 17 9x19mm comes out to 1d+2 pi, with some overflow on the pi multiplier we could use to justify 2d-1, or perhaps even 2d. As a pistol firing a pistol-calibre bullet, it has an easy AD of (1). At an average of 6 damage versus an average human character, it would take an average of 3.33~ shots to force a death check; compared to the unmodified weapon at 2.22~ shots. Based on my understanding of real-world incidents, 3 1/3rd shots seems more "reasonable/realistic" than a hair over 2 1/5th.

Variants on the same calibre, shot out of the same gun, can then be handled by the bullet modification rules.

I await a multitude of corrections from those who actually know what they're talking about. ;)
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Old 12-11-2020, 06:38 PM   #2
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Default Re: Base gun damage on calibre and gun size; armor divisor on kinetic energy?

Well, the Glock 17 in 9x19mm does 2d+2pi by definition: that's a calibration point for the GURPS damage scale, but changing your calculations to be based on that will not be difficult.

Apart from that, I am not sympathetic to the "Survivable Guns" reasoning. We have some real-world data points for rifles, which don't seem to match up with it. In wars, quite a few people get shot with rifles and machine guns, and they are normally seriously wounded or killed outright. Also, large animals such as cattle, horses, and even elephants don't have much armour, and can be killed quite readily with a few rifle hits.
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Old 12-11-2020, 06:53 PM   #3
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Default Re: Base gun damage on calibre and gun size; armor divisor on kinetic energy?

It's reasonable to base wound channel width on caliber, but GURPS already represents this in the pi-/pi/pi+/pi++ distinction. However, energy determines depth of penetration in tissue, and until the bullet passes all the way through, penetration matters a lot.

There's actually a pre-existing rule that handles this, however. Just use blowthrough, basing it on damage before wounding modifiers.
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Old 12-11-2020, 07:13 PM   #4
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Default Re: Base gun damage on calibre and gun size; armor divisor on kinetic energy?

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Originally Posted by johndallman View Post
Well, the Glock 17 in 9x19mm does 2d+2pi by definition: that's a calibration point for the GURPS damage scale, but changing your calculations to be based on that will not be difficult.
Noted. Also a bit humorously ironic. :)
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Originally Posted by johndallman View Post
Apart from that, I am not sympathetic to the "Survivable Guns" reasoning. We have some real-world data points for rifles, which don't seem to match up with it. In wars, quite a few people get shot with rifles and machine guns, and they are normally seriously wounded or killed outright
I would guess most of those statistics come from WWI and WWII, which are TL6-7. As such, being in the real world, metaphorically speaking, all of the "brutal realism" switches would be "on", including bleeding. In addition, most wounds to the torso should, realistically speaking, be tracked as Vitals hits (lungs or heart).

Running the Springfield M1 Garand in .30-06 through my guestimate, using a new baseline of the Glock 17, 9x19mm, we get 2d-1 pi, with enough left over on the pi multiplier to bump that to 2d or 2d+1 (depending on how generous the conversion from left-over pi multiplier to damage is). With 7 damage to the torso, and acknowledging most torso hits as probably also vitals hits, that's 21 damage; enough to force an instant death check.

Add in bleeding rules, and they're making a bleeding check at -4 every minute, assuming they do nothing. With a hit to an arm or leg, 7 damage is enough to cripple the limb of an average human.
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Also, large animals such as cattle, horses, and even elephants don't have much armour, and can be killed quite readily with a few rifle hits.
I don't have enough info to even begin discussing this, save to say that the cube in square-cube law, is probably a not a great measure of hit points for a biological creature. After all, making a human twice as tall, wide, and thick, doesn't multiply the intervening mass between their vital organs and their skin by x8, but rather, by x2.
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Old 12-11-2020, 07:21 PM   #5
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Default Re: Base gun damage on calibre and gun size; armor divisor on kinetic energy?

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Originally Posted by Anthony View Post
It's reasonable to base wound channel width on caliber, but GURPS already represents this in the pi-/pi/pi+/pi++ distinction. However, energy determines depth of penetration in tissue, and until the bullet passes all the way through, penetration matters a lot.

There's actually a pre-existing rule that handles this, however. Just use blowthrough, basing it on damage before wounding modifiers.
I couldn't find "blowthrough" in the Basic Set; I'm going to guess its in Tactical Shooting? I don't have that one (yet). I have heard of it before as some sort of limit on the damage a gun can do, based on the target's total HP.

A good point on depth of penetration/blowthrough; however, this would seem to somewhat render all guns the same, past a certain point determined by the target's hit points and armour. That's not to say that it isn't the realistic solution.
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Old 12-11-2020, 09:38 PM   #6
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Default Re: Base gun damage on calibre and gun size; armor divisor on kinetic energy?

That rule would also remove any difference in damage between say a .38 Special from 1910 and a 9 x 19 mm Parabellum with the latest high-powered load in 2010.

Quote:
Originally Posted by Say, it isn't that bad! View Post
I couldn't find "blowthrough" in the Basic Set; I'm going to guess its in Tactical Shooting? I don't have that one (yet). I have heard of it before as some sort of limit on the damage a gun can do, based on the target's total HP.
GURPS 3rd edition had a Blowthrough rule where base damage above a threshold depending on body part is ignored. GURPS 4th edition has an Overpenetration rule (p. B408) but the only one for ignoring excess damage is on page B421 "A blow to a limb or extremity can never cause more injury than the minimum required to cripple that body part. For example, if a man has 10 HP and suffers 9 points of injury to his right arm, he loses only 6 HP the minimum required to cripple his arm."
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Old 12-11-2020, 10:49 PM   #7
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Default Re: Base gun damage on calibre and gun size; armor divisor on kinetic energy?

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Originally Posted by Polydamas View Post
GURPS 3rd edition had a Blowthrough rule where base damage above a threshold depending on body part is ignored. GURPS 4th edition has an Overpenetration rule (p. B408) but the only one for ignoring excess damage is on page B421 "A blow to a limb or extremity can never cause more injury than the minimum required to cripple that body part. For example, if a man has 10 HP and suffers 9 points of injury to his right arm, he loses only 6 HP the minimum required to cripple his arm."
Looking at the rules for "cover DR" on B408, it looks like you could use cover DR from HP as a maximum to "piercing, impaling, or tight-beam burning" damage (not injury). For limbs, using the amount of damage needed to cripple them as "cover DR" therefore seems "sound enough".
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Old 12-12-2020, 12:04 AM   #8
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Default Re: Base gun damage on calibre and gun size; armor divisor on kinetic energy?

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Originally Posted by Say, it isn't that bad! View Post
Looking at the rules for "cover DR" on B408, it looks like you could use cover DR from HP as a maximum to "piercing, impaling, or tight-beam burning" damage (not injury). For limbs, using the amount of damage needed to cripple them as "cover DR" therefore seems "sound enough".
I think that was how the old 3rd edition Blowthrough rule worked, and the authors removed it in 4th edition for reasons (like they removed the old rule for automatic decapitation above a certain amount of Cutting damage to the Neck).
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Old 12-12-2020, 12:19 AM   #9
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Default Re: Base gun damage on calibre and gun size; armor divisor on kinetic energy?

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Originally Posted by Say, it isn't that bad! View Post
I couldn't find "blowthrough" in the Basic Set; I'm going to guess its in Tactical Shooting? I don't have that one (yet). I have heard of it before as some sort of limit on the damage a gun can do, based on the target's total HP.
You want High Tech p. 162. Optional Wounding Rules box, heading Body Hits.
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Old 12-12-2020, 12:46 AM   #10
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Default Re: Base gun damage on calibre and gun size; armor divisor on kinetic energy?

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Originally Posted by Polydamas View Post
That rule would also remove any difference in damage between say a .38 Special from 1910 and a 9 x 19 mm Parabellum with the latest high-powered load in 2010.
It also seems to suggest (assuming I'm reading it right) that an AP-74 in .32 ACP would do the same damage as an M-14 in 7.62 NATO, which is just a completely nonsensical result.
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