Seeking Stats: High-tech Air Gun

[DataPacRat]

For a post-apocalypse setting, I'd like to have some reasonable and/or realistic statistics handy for a particular object: an airgun pistol. My usual sources for generating close-enough stats (GURPS 3e's Vehicles, "Guns Guns Guns") don't seem to cover compressed-air weapons, so I'm hoping someone here has some relevant knowledge or links.


Detail One: Air tank material. Pre-apocalypse tech included a 3D-printed material which, when used for Halloween costume armor, was strong enough to take a near-point-blank long-arm round to the chest with no more harm than a small divot and the wearer being knocked back. I'm presuming that the existence of such a substance implies material science that could produce air tanks holding more pressure than present-day tanks are capable of.

Detail Two: The weapon is to be manufactured by a surviving pre-apocalypse "mini factory", primarily a set of 3D printers, which is programmed to obey local laws about what is and isn't allowed to be made. Gunpowder-based firearms are prohibited; airguns aren't.

Detail Three: The protagonist is looking for one or more holdout weapons to start with, possibly starting with something close to the form-factor of a Kel-Tec P-32, or a similar mouse gun, if that's feasible. Other forms aren't ruled out, though, such as pepper-box derringers or palm pistol or single-shot zip-gun or whatnot.


Given these details... what are your thoughts?

(Bonus points for any suggestions on how to Munchkin the thing in-universe, such as using the air tank as an emergency air supply or 'not quite a bomb'... :) )

[whswhs]

Aren't there air guns in GURPS High-Tech? I'm pretty sure I saw some there last I looked through it.

Bill Stoddard

[DataPacRat]

Originally Posted by whswhs

Aren't there air guns in GURPS High-Tech? I'm pretty sure I saw some there last I looked through it.

Bill Stoddard

GURPS 4e's High-Tech only seems to have stats for air-rifles and a paintball gun. 3e's High-Tech has a couple of stats on page 44 for a generic 1950 AD sporting air pistol - damage and range, mainly. Nothing about weight, how much air is held in the reservoir, how the stats might change for a 2050 AD version or a non-sporting version, if any tradeoffs can be made with air pressure versus bullet velocity versus caliber, or any other crunchy goodness.

[Frost]

Originally Posted by DataPacRat

For a post-apocalypse setting, I'd like to have some reasonable and/or realistic statistics handy for a particular object: an airgun pistol. My usual sources for generating close-enough stats (GURPS 3e's Vehicles, "Guns Guns Guns") don't seem to cover compressed-air weapons, so I'm hoping someone here has some relevant knowledge or links.

I don't know if you have already ruled them out or if they would meet your requirements for realism but there are a few compressed gas weapons in the 4e version of Ultra tech. The list does already include a bulk -1 holdout weapon (although it is rather anaemic) although you might be better off trying to back engineer from the examples to get what the player wants.

[Flyndaran]

Talking partially out of my rear:

I think the force expelling a projectile follows air pressure in a linear fashion. Twice pressure = twice force, everything else being equal.

Twice the pressure means twice the volume of air compressed into the same container.

I think that as material sciences advance, the maximum safe pressure should increase linearly. Twice the material strength = twice the safe pressure.

But ultra tech air rifles would make very nasty grenades if damaged. As well as take forever to re-compress with animal power alone.

[Fred Brackin]

[QUOTE=DataPacRat;1831591Given these details... what are your thoughts?

[/QUOTE]

Based on what I've seen even from some extreme TL8 air guns you'd get nothing better than half the damage of a same-sized smokeless powder pistol. There probably is no level of compression where you'll get results comparable to modern gunpowder.

A significant part of the problem would be your compressed gas starting at room temperature. Then when you release it, it cools. This cooling fights against expansion.

Also, the highest compression I've heard of in TL8 working gear is 300 atmospheres in Navy submarine tanks. That sounds like a lot but it's only 4500 psi which is at the bottom end of old black powder pressures. I don't think even pure lead obturates below 5000 psi.

Then there's com pressing ambient air might sound convenient for a post-apocalyptic type but air is far from an ideal gas. Compress it enough and you'll start seeing endothermic reactions like the formation of nitrogen oxides. Your compression almost certainly stops being effective before then. Probably you start adding more heat which radiates away from the storage tank than youy get stored energy.

The best weapon I could design using hard science would be like the UT air guns i.e using a sharp projectile and a payload rather than raw KE.

[DataPacRat]

Originally Posted by Frost

I don't know if you have already ruled them out or if they would meet your requirements for realism but there are a few compressed gas weapons in the 4e version of Ultra tech. The list does already include a bulk -1 holdout weapon (although it is rather anaemic) although you might be better off trying to back engineer from the examples to get what the player wants.

While I was trying to play with those stats, the 3mm needlers were the best of that bunch, though I couldn't figure out any good rationalizations for how to tweak the stats, either to compress the standard needler into holdout size or give the wrist needler a proper grip instead of the wrist-mounted silliness.

Originally Posted by Flyndaran

But ultra tech air rifles would make very nasty grenades if damaged.

The character is sufficiently Munchkinesque to treat that as an opportunity.

Originally Posted by Flyndaran

As well as take forever to re-compress with animal power alone.

Mechanical air pumps can be made, as can biodiesel and other power sources; the restriction on gunpowder-style weapons is a legal/software issue.

Originally Posted by Fred Brackin

Based on what I've seen even from some extreme TL8 air guns you'd get nothing better than half the damage of a same-sized smokeless powder pistol. There probably is no level of compression where you'll get results comparable to modern gunpowder.

An equivalent to a .22 mousegun would be perfectly acceptable. While more powerful rounds might be better still, it's not like anyone /wants/ to be shot with a .22.

Originally Posted by Fred Brackin

A significant part of the problem would be your compressed gas starting at room temperature. Then when you release it, it cools. This cooling fights against expansion.

Also, the highest compression I've heard of in TL8 working gear is 300 atmospheres in Navy submarine tanks. That sounds like a lot but it's only 4500 psi which is at the bottom end of old black powder pressures. I don't think even pure lead obturates below 5000 psi.

Then there's com pressing ambient air might sound convenient for a post-apocalyptic type but air is far from an ideal gas. Compress it enough and you'll start seeing endothermic reactions like the formation of nitrogen oxides. Your compression almost certainly stops being effective before then. Probably you start adding more heat which radiates away from the storage tank than youy get stored energy.

Machinery is around to create dry ice and liquid nitrogen for simple cryonics; is there a particular ambiently-available gas that would serve better for compressed propellant than others, if cost isn't an issue?

Originally Posted by Fred Brackin

The best weapon I could design using hard science would be like the UT air guns i.e using a sharp projectile and a payload rather than raw KE.

This sounds a lot like an up-teched blowgun, which isn't a bad thing. There's been a lot of interesting biological organisms, including aconitum where there was no aconitum before. Any particular GURPS stats for that toxin, or shall I just wing it with Wikipedia?

[Anthony]

Originally Posted by DataPacRat

The character is sufficiently Munchkinesque to treat that as an opportunity.

Nasty, in this case, means 'way less powerful than a real grenade, but less stable than ammunition'.

In terms of stats, you can probably do okay by taking an existing pistol and halving its damage.

[johndallman]

Originally Posted by DataPacRat

is there a particular ambiently-available gas that would serve better for compressed propellant than others, if cost isn't an issue?

Hydrogen seems to have some potential.

[Anthony]

Originally Posted by johndallman

Hydrogen seems to have some potential.

Though it usually gets compressed by explosives...