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Old 04-01-2017, 08:07 PM   #9
Join Date: Aug 2004
Location: Buffalo, New York
Default Re: GURPS ULTRATECH ballistics?

Originally Posted by DouglasCole View Post
Likely. There are a lot of funky units at play here. Grains, grams, pounds . . .

The 1/2D calculations used an exponential fall off of velocity based on sectional density. It's close enough to give a good approximation in the supersonic regime but isn't nearly as good as a real ballistics table.

the gas expansion stuff is just a representation of how much energy goes into the bullet, based on integrating force over distance. I used the ideal gas law to figure out how the pressure falls off as the volume expands (the volume is the chamber plus the barrel) and then just integrated Force = Pressure x bullet area over the barrel length.

Expansion is something I just plug into my penetration and wound channel equation - increase bullet caliber by the expansion ratio, and let the rest take care of itself.

For basic bullets thrown from ultra-tech guns, you're doing the right idea - take best guesses for caliber, mass, and velocity, and adjust mass and velocity so they're not crazy-town and the damage is about right. The assumptions going into UT were not all rational and logical and if A then B, so it might not work every time.
Thanks for the comments Doug. I kind of knew that any time you have a final damage value that is based on Mass times velocity Squared, I knew that this would be a function value where you fill in 1 value, the remainder would automatically spit out.

My next post is going to be based on some conjecture, and I will use the historical data for a 7.5mm round gun from the past designed in 1883, using black powder (presumably) that was used until 1903 in the Swiss army. Presumably, that round had satisfactory characteristics for flight, that it was worth retaining. Its mass was 110 grains (range given is 102 to 110, so let's use the upper for this example).
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