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Old 04-01-2017, 01:43 PM   #1
Join Date: Aug 2004
Location: Buffalo, New York
Default GURPS ULTRATECH ballistics?

Hello Folks,
I've been promising myself that someday, I'd examine the rules for liquid propellants for use in Ultratech weapons of the future. Today, seems to be that day.

First, the background:
Douglas Cole produced a spreadsheet that gave a formula for approximating cartridge damage from real world data. From that original spreadsheet, I revised it for my own needs so I could simply enter in the bullet weight by grains, velocity by feet per second, diameter in inches, and it would give me a GURPS 4e bullet damage value. It was so good, that when I creaed a database for black powder weapons of various bullets in grain weight, plus velocity of bullet achieved as muzzle velocity - I was able to compute the GURPS damages that largely matched what was in the GURPS ADVENTURE GUNS supplement.

So, let's take a historical cartridge listed in my database. It is the .25-25 Stevens center fire cartridge produced in 1895 with a 66 grain bullet and a muzzle velocity of 1180 feet per second. The estimated muzzle energy seems to have been 204 foot pounds per the calculations.

In GURPS terms, such a weapon would have a 2d6 damage rating.

So, how do back track the stats for a bullet as if this weapon were using liquid propellants (in order to determine what the weapon's muzzle velocity has to be to achieve the damage listed in the rules as listed below:

"Boosted velocity dumps extra propellant into the firing chamber: increase the piercing damage by +1 per die and multiply range by 1.3. Each counts as 1.5 shots for purposes of draining the propellant bottle."

So, a weapon that normally does 2d6 damage, with a 66 grain bullet, has to have a velocity of 1180 feet per second - regardless of whether it was produced by black powder, modern gun powder, or even liquid propellant. In order to achieve an average damage of 9 points instead of 7, we'd need to increase the velocity if we keep the grain weight the same.

So how much of a velocity increase are we talking about?

We'd need to get the velocity from 1180 up to 1490 feet per second. Those of you who have a better grasp of the "joules per" weight measure or joules per volume measure of the propellant can chime in, or you can perhaps say "Hey Hal, nice idea in theory, but... that doesn't match my calculations."

So, showing all work:

ft/lbs (cell A57) =H57*I57^2/450395

Data in cells H57 was weight of bullet in Grains, and I57 was Velocity in feet per second (in this case, 66 grains and 1180 feet per second)

ME in J. (cell B57) =1.35581794833189*A57 (or Ft/lbs value)

dam pts (cell C57) =ROUND(SQRT((B57^1.04)/((PI()*((E57/39.37)/2)^2)^0.314))/13.3926,1)

E57 is bullet diameter in inches, B57 is Muzzle energy in joules.

Damage Dice (Cell D57) =CONCATENATE(INT(C57/3.5),"d+",FLOOR(3.5*(C57/3.5-INT(C57/3.5)),1))

C57 = average bullet damage

Bullet Diameter in inches (Cell E57) =0.25
Bullet weight in Grains (Cell H57) =66
Bullet Velocity in feet per second (Cell I57)=1180

Those were all based on the spreadsheet that Douglas Cole built. Hope this helps people to visualize their stats for how the weapons function.

Using a modern day bullet such as the .44 colt with its 210 grain bullet, 660 feet per second velocity, and its listed muzzle energy of 206, when I plunk those values into the spreadsheet, I get 1d6+2 damage (modern pistol round). The 45 ACP on the other hand, when entered into the spreadsheet, shows that for a round listed as being a jacketed hallow point at 185 grains, 1000 feet per second muzzle velocity - that would be, per the spreadsheet, roughly 2d+1.

Now the fun part would be - to try and figure out what the bullet weights of the Caseless Ammo in GURPS ULTRATECH are by weight, and figure out what the muzzle velocity has to be based on the damage ratings given in GURPS ULTRATECH.

The 7.5mm caseless ammunition is roughly a .30 caliber gun. Using the 7.5mm Swiss Army Revolver Stats for the bullet (from CARTRIDGES OF THE WORLD), that's a 110 grain bullet. The muzzle velocity of the gun, in order to achieve a 2d6+2 damage rating, would require a velocity of 1380 feet per second.

In theory, it should be possible - using the stats above, to come up with a formla that gives us the following:

Bullet weight in ULTRATECH - which is supposed to be the combined weight of the bullet plus its propellant and packaging..
Caseless ammo is what - a ratio of bullet weight to propellant? If for instance, the weight of a round is 1/3rd bullet weight, and 2/3rds propellant - we should be able to get the joules per mass of a given propellant in order to be able to custom build bullets for use with GURPS ULTRATECH. Using real world bullet weights of modern bullets, and then using the Joules per Gram measurement or Joules per grain (I know I'm mixing Imperial with Metric darn it!) - we should be able to specify the stats for a 7.6mm round, a 5.56mm round, or a 11.5mm round easily enough.

Just thinking...
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