Old Ballistics Spreadsheet: A Repost

A long time ago, I wrote an article that began my dark journey into the crunchiest of crunch. It was my first article, and my first professional gaming-related publication.

Interior and Terminal Ballistics for GURPS

The goal was to take the smallest set of inputs and get to a GURPS damage value, matching a whole bunch of parameters to the penetration of RHA steel where I could find numbers. Most of those numbers were in the smallarms range, which is fine for most adventuring needs. I did peg the way, way upper end based on the M829A1 tank projectile (I think at the time, there was no A3 and the A2 was really hard to get data on) as well as the 406mm projectiles from the Iowa-class battleships. Turns out, that you really only need the kinetic energy of the projectile and the caliber of the impactor to get a “good enough” match to the real world numbers for consistency in gameplay.

I was pretty focused on precision over simplicity when I first wrote the thing, nearly 20 years ago. That led to:

Damage (points) = sqrt(KE^1.04/Xsect^0.314)/13.3926

Ugly.

But there’s really no need to get THAT precise, so the latest version is something a bit more friendly:

Damage (points) = sqrt (KE/(Caliber (mm) ^0.4))*0.6

This is much “nicer,” without actually crossing over the threshold to either “nice” or “simple.” The old Xsect (cross-section) figures were a mess. That’s something you can probably just do with a pocket calculator, and it’s again close enough. It also had the side-effect of adjusting some of the numbers that straddled breakpoints downward a bit, which brought them closer in line with published values. That was nice.

Since then, I created and published a spreadsheet to help with the interior ballistics part, and while it’s not simple, it does get the job done, more or less.

Ballistics Model Dec-2020 <–(this is a link)

I’ve seen a bit of a flurry of activity asking for older copies of the sheet, so I thought I’d repost it with the update.

The point of this is mostly to allow a world-builder to guesstimate the performance of oddball or new projectiles, or answer consistently (and for a game, consistency can be more important that accuracy) “what would this rifle/pistol do if I shortened/lengthened the barrel?”

That’s basically the entire point of The Reloading Press, mostly recently demonstrated taking a peek at the .277 SIG Fury, an 80,000 psi cartridge that’s a contender for the US Army’s Next Generation Squad Weapon program.