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The general measure for MD seems to be mapped to Kinetic Energy, which makes sense. However, as I was looking over siege weapons, there is a bit where it says that they should be stay SDC. This is also understandable, but at what point should momentum be used to map damage for things like ramming with ships, or huge trebuchets or other large momentum events?

-STS

I usually keep things like that a cinematic GM decision, kinda like when rocks fall and all the players are dead. no rules really cover it, so instead, you make it a problem to motivate player action, part of the background like an earthquake causing the ground to open up and players falling in to move a scene, etc. have their ship be rammed, go through a boarding action, and after the ship is stuck untill the crew can repair it, forcing the players to explore for materials or some such.

The problem with the kinetic energy is that it doesn't scale with weapons unless the ratio you use is the square root of the kinetic energy:

• .22 LR: 178-191J, 259J with 30-grain HP, 277J with 31-grain RN; treat as "X"
• .380 ACP (aka 9x17mm or .380 Auto): 275-280J, 405J Buffalo +P HC-FN, 419J Norma MHP, 457J RBCD TFSP round; 1.5X standard, 2.1/2.2/2.5X for the specials
• 9mm Parabellum: 481-494J normal, 675J Norma ENVY FMJ, 679J Underwood JHP +P+, 729J RBCD TFSP round; 2.6X (2.6-2.67X) standard, 3.6-3.9X for specials
• .45 ACP: 483-559J normal, 639J Double Tap, 803J Underwood JHP +P+, 1124J RBCD TFSP round; 2.8X standard (2.6-3.0X), 3.4/4.3/6.1X specials
• .40 S&W: 628-635J normal, 680J Glaser, 660J Double Tap FMJ FP, 797J Underwood JHP; 3.4X standard, 3.7/3.6/4.3X specials
• 5.56x45mm NATO: 1,679-1,859J; 9.1-10.0X
• 7.62x51mm NATO: 3,470-3,590J; 18.8-19.4X
• .50 BMG (12.7x99mm): 17,952-20,195J; 97.0-109.2X

If kinetic energy was straight-line, then using 1D6 SDC for the .22 LR gives you OK damages for the pistol cartridges (2D4 or 2D6 for .380 ACP, 2D6-3D6 for 9mm Parabellum, 3D6-4D6 for .45 ACP, & 3D6-4D6 for .40 S&W), but the rifle cartridges get ridiculous (1D6x10 SDC for 5.56mm, 2D6x10 SDC for 7.62mm, & 1D6x100 SDC/1D6 MD for unmodified .50-caliber).

This is why if you're going to use kinetic energy, you almost have to use the square root of the change in kinetic energy:

D1 = D2 x (KE1 / KE2) ^ 0.5

Pistol cartridges are somewhat reasonable but end up being a little underpowered (1D6 SDC for .22 LR ratios to 2D4 SDC for .380 ACP and 9mm, 2D6 SDC for .45 ACP & .40 S&W), although the rifles are a bit more standard (3D6 SDC for 5.56mm, 4D6 SDC for 7.62mm, 1D6x10 SDC for .50 BMG)...but even those end up a bit on the low side.

Momentum is probably the better way to go. And since both the mass & velocity are to the power of 1, you should be able to use a straight ratio:

D1 = D2 x (m1 / m2)

But then you get some whacky results for the 7.62mm & .50 BMG cartridges (1D6x10 and 4D6x10-5D6x10 SDC, respectively).

So I'm not sure if there's a really good mathematical way to represent it. I would personally recommend Kitsune & Mischa's Netbook of Modern Firearms, if for nothing else than the modified damage-per-cartridge chart (which includes a lot of other cartridges besides the standards).

'momentum is measured by speed and weight in the PB game system. They are covered in HU RPG 2nd ed core-book in the vehicle combat section and in the HUGMG in the Rampage section. I don't have where they are covered in Rifts at the top of my head.

The problem with these is that the damage ratings are mainly separated and done by speed or by weight. There is no integrated damage rating table that I can remember of OtToMH.

Well, for my KE based damages I use is the square root of (Muzzle Energy in ft lbs) x the expanded diameter of the bullet). Using the KE accounts for the differing velocities through different barrel lengths, so it allows for a lot of precision in DC calcs. The bullet expansion allows me to get some differentiation between bullet types such as FMJ and HP or what not. This seems to map fairly well to most weapons where the impact area is less than 1 square inch.

Larger impacts it gets iffy, and that was where I was wondering about momentum as a measure of damage capacity.

I have used Kitsune's stuff for a long time.

-STS

The problem with Rifts is that, unlike HU which does have rules on crash/impact, MDC vehicles sometimes show mega-damage values for ramming, which makes no sense as a SDC vehicle going at the same speed would inflict only standard damage, not mega-damage. It's not like MDC vehicles are zooming around at 100x the speed. Just another case of the Rule of Cool driving rule inconsistency. That's why it's up to GMs to make the call in Rifts within the context of the story.

Ok,
so with some napkin math, my idea for damage based on Momentum is the cube root in Kg m/s.

So 1000 kg m/s is 10 damage. That maps decently onto the "humans have 9 HP" argument in the D&D community.

So, these are my two equations for damage for momentum (impact injuries) and for KE (penetrating injuries).

mass x velocity = momentum in kg m/s ^1/3 = damage
1/2 mass x velocity ^2 = KE/diameter of impactor = damage

-STS