"Quick and Dirty" Barrel Length & Damage?

[Tallor]

If I were to write up a series of guns (small and cannon) based roughly on Ultra-Tech and Spaceships, would there be changes in range and damage with longer/shorter barrels? I'd like to make snub-nosed infantry support vehicles and long-nosed mobile artillery, but I don't have the math.

Any suggestions?

[clu2415]

Barrel length affects velocity. For artillery and direct-fire explosive shells, damage there comes from the explosion; the velocity affects how far you can fling your payload, but as long as the payload gets there, it will do the same thing regardless. For direct-fire, kinetic energy weapons, velocity would affect damage.

How much velocity is lost is another question. Check out this data the Swedish military collected about 5.56 velocities. Cutting off 28% of the barrel reduced the velocity by 7%. That's about the limit of my ballistics knowledge.

[Purple Haze]

Longer barrels tend to produce higher velocities, but a too long barrel can reduce them.

A lot is dependant on the speed of the powder. A slow powder in a short barrel makes a huge muzzle flash and wastes a lot of energy compared to a fast powder. In a long barrel the slow powder either contains more energy or transmits it to the bullet more efficiently.

There really is no way of knowing what lengthening or shortening a barrel will do without knowing everything there is to know about the round being fired.

[Varyon]

If you're scaling the propellant loads (longer barrels use more propellant), damage is roughly linear with length (at least, going with the rules from 3e Vehicles). If you're still using the same rounds with a different length barrel, however, you're basically looking at a case of making a sawed-off weapon - TS69 has rules for how Bulk and Damage gets modified there.