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Old 04-01-2017, 08:51 PM   #17
hal
 
Join Date: Aug 2004
Location: Buffalo, New York
Default Re: GURPS ULTRATECH ballistics?

OK, the numbers for real world damages look like they're pretty good, which means that for conventional firearms in an UT world, they should work as well.

After hitting GURPS CLASSIC MODERN FIREPOWER, the stats for the 4.73 Caseless Telescoped Ammunition indicate that it weighs .011 pounds per round, which works out to be roughly almost 5 grams in weight. Damage for the round is listed as being 4d6+2. Real world data suggests a muzzle velocity of 930 meters per second. The overall weight of the bullet is 3.25 grams. It would appear that the remaining 1.73 grams of weight is the actual propellant.

At 930 meters... that's about 3051 feet per second.

Ok, filling in those values for the spreadsheet, I end up with 3d6+2 damage (50 grains bullet weight, 3051 feet per second velocity). GURPS HIGH TECH for 4e shows that the assessed damage is still what it was in GURPS CLASSIC Modern Firepower: 4d+2.

Either Hans had his reasons for bumping up the average damage value by a full 3.5 points, or he didn't have access to this spreadsheet when he compiled the damage (or some other possibility that I've not considered).

In any event, the propellant weight of 1.73 grams will produce roughly 1401 Joules of energy. This works out to be roughly 809.8 joules of energy per gram of propellant.

This presumes of course, that the full energy available is 100% utilized, which I suspect is not the case, but it gives me a useful approximation of what should be the situation.

Now, Lead has a specific gravity of 11.34 grams per cubic centimeter? With a mass of 3.25 grams, the volume of the bullet is about .28 cubic centimeters. If the density of the propellant materials is close to that of water, then the volume of propellant is roughly 1.73 cubic centimeters to the .28 cubic centimeters of the round itself. That's roughly a 6;1 volume ratio for the propellant to projectile.

so, let's re-examine the UT 7.5mm round. Clearly, a 110 grain round is a bit much, since a 4.73 round is only 50 grains. What would be a valid bullet weight for a 7.5mm round?

Last edited by hal; 04-01-2017 at 08:54 PM. Reason: Missed the word in blue that makes all the difference
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