Joined: 18 Jul 2011
|Posted: Tue Jun 19, 2012 1:22 pm Post subject: Weapon damage conversion from penetration/GURPS stats
|Hey, all: putting this in a new thread since the old one sort of died on the vine. I've come up with what seem to be reasonable conversions from the RHA penetration of weapons and/or GURPS damage to Savage damage and AP, which may be useful to those adapting weapons not in the existing material.
Things get a little ad hoc below about 1/2 inch penetration or 10d damage in GURPS terms, unfortunately, but for the most part this works pretty well.
Essentially, if we use the 2.5 Armor = 1 RHA rule given in the vehicles section, it's fairly easy to reach stats comparable to those given for existing weapons. We use the .50 BMG as a stepping off point. I figure 12 average damage is fairly accurate in the system, and while the AP is technically too high for the RHA rule to work out exactly there's a bit of an AP floor required to account for penetration against lightly armored targets, so that's just something you have to offset. In general, then, a weapon's average damage and AP can be calculated from (RHA penetration(in.) * 2.5 or GURPS damage dice/. Add to 10 for average damage (and then split to appropriate dice) and add to 2 to determine AP; the weapon is a Heavy Weapon. If the damage/penetration is less than 1, figure the damage as 12 - each .25 or less of penetration/dice of damage, and from AP at .5 and .25.
To demonstrate: the .50BMG cartridge penetrates approximately .6 in of RHA or does 12d in GURPS terms. Hence, the damage becomes 10 + (.6 * 2.5 or 12/ = 11.5, rounding up to 12 (so 2d10) and penetration is 2 + 1.5, rounding up to 4 and nicely matching the given stats.
As an RHA example: the penetration of the 30mm GAU-8 on the A-10 Avenger is approximately 2in. Figuring from this, we get a damage of 10+5 and AP of 2 + 5; 15 gives us 3d8 damage (figuring for an average damage of 6 per acing d and AP 7. This fits pretty well with the role of killing light armor and occasionally tanks (front armor on a BMP-2 is 16(4), if I recall, while rear armor on an Abrams is I think 29(12);
it should have a good shot at any angle on the APC and might be able to damage the Abrams on a good shot. This is a case where I think the High Cyclic Rate rules from Modern Ops work nicely, as you hit the least armored area and get +4 to hit/damage, making a raise and an average of 23 vs. 29-7 = 22 Toughness).
In GURPS terms, you'll want to use the damage type and armor divisor multipliers to figure penetration and damage separately. As an example, the .45 ACP has a 1.5x damage multiplier, so it should do the equivalent of 3d or .375 damage; as this is between .25 and .5 an average of 12 - 3 = 9 seems appropriate (2d6+1), but the 2d damage at .25 unmultiplied implies that there should be no AP. Conversely, a 125mm APDS tank round does about 200d damage with a (2) armor divisor; this should give it an AP of 52 (400/ and a damage of 4d12 - 1 (27, rounding up). Conveniently, this about bounces off the front of an Abrams on average, which is likely the design target for the front armor.
One difficulty in the RHA conversion is accounting for projectile size effects (e.g., a penetrator or shaped charge probably does not do as much damage as raw penetration would suggest). I think the quickest way, and an in general way to handle AP rounds, is probably to use the effective AP but reduce the die type by one from the calculation; e.g., if it were decided the rounds in the 30mm gun were considered special AP rounds, we would figure at AP 7 but reduce the damage down to 3d6. For existing weapons, double the AP but one less die type seems about right, or +4 AP if you want to stick with the trappings equivalent for armor piercing in SWD.
A related question: how to handle expanding or frangible rounds? The SWD power trappings suggest the Shard trapping (+1 damage against unarmored, -1 against armored) but alternatively half AP and +1 die type against unarmored, -1 die type against armored seems sensible too.
Thoughts? Is this useful to anyone?