How to Stat Ghost in the Shell Style Hypervelocity Ammunition

[Rupert]

Originally Posted by GURPS Fox

As far as I know, the absolute limit for 'conventional' gunpowder (i.e., nothing that uses things like electrothermal chemical propellants) is 1.8km/s.

It has to be higher than that, because modern tank guns reach that and they have massive muzzle blasts, so there's plenty of energy and pressure still in the propellant gasses.

The Russians are claiming a little over 2 k/s for their new 125mm gun and ammo, but that being hype and exaggeration is quite possible.

[GURPS Fox]

Originally Posted by Rupert

It has to be higher than that, because modern tank guns reach that and they have massive muzzle blasts, so there's plenty of energy and pressure still in the propellant gasses.

The Russians are claiming a little over 2 k/s for their new 125mm gun and ammo, but that being hype and exaggeration is quite possible.

From what I gather, by 1.8km/s you're getting inefficient when it comes to things like powder burn (think the M551 Sheridan's caseless round problem, the powder didn't burn efficiently enough, and it caused problems like cookoff while you load a new round in). In addition, you could only get 2km/s with sabots, and even then, you'll start to get inefficient.

So, here is an 'HV Ammo Ruleset' I've cooked up with your guys' input:

• HV Ammo is only available to 'conventional' guns built using tech from TL8 or later.
• HV-compatable weapons have a 1.1x weight multiplier to simulate the added features to make the weapon take the pressures and punishment of an HV round.
• In ANY Non-HV TL8 or higher weapon that uses HV Ammo, increase the Malf. by two steps in the initial magazine and increase Malf. by one step per additional magazine. If the weapon is made at tech levels less than TL8, turn the weapon and magazine into a grenade of similar weight (to simulate a catastrophic cook-off).
• Base ammo (i.e., FMJ/Solid) is 1.25 times the damage and range. All rounds have a 1.5x modifier to the cost of the round. Example: a 5.56NATO round from an HV-compatible M16 rifle will get a damage roll of roughly 6d+1 instead of the usual 5d.
• Other modifiers (like ETC or saboted ammunition) can be applied to HV ammo.

[The_Ryujin]

Originally Posted by GURPS Fox

From what I gather, by 1.8km/s you're getting inefficient when it comes to things like powder burn (think the M551 Sheridan's caseless round problem, the powder didn't burn efficiently enough, and it caused problems like cookoff while you load a new round in). In addition, you could only get 2km/s with sabots, and even then, you'll start to get inefficient.

So, here is an 'HV Ammo Ruleset' I've cooked up with your guys' input:

• HV Ammo is only available to 'conventional' guns built using tech from TL8 or later.
• HV-compatable weapons have a 1.1x weight multiplier to simulate the added features to make the weapon take the pressures and punishment of an HV round.
• In ANY Non-HV TL8 or higher weapon that uses HV Ammo, increase the Malf. by two steps in the initial magazine and increase Malf. by one step per additional magazine. If the weapon is made at tech levels less than TL8, turn the weapon and magazine into a grenade of similar weight (to simulate a catastrophic cook-off).
• Base ammo (i.e., FMJ/Solid) is 1.25 times the damage and range. All rounds have a 1.5x modifier to the cost of the round. Example: a 5.56NATO round from an HV-compatible M16 rifle will get a damage roll of roughly 6d+1 instead of the usual 5d.
• Other modifiers (like ETC or saboted ammunition) can be applied to HV ammo.

Looks Good. HV guns should also weight at lest 1.1 and cost 1.5 to 2× as much to cover the stronger barrel and chamber.

[hal]

Been away for vacation and am back - will look into what goes into a Hypervelocity 9mm round in order to get a given damage value based upon:

a) a given bullet mass
b) a given velocity range

Will likely do this sometime tomorrow...

but just to give you an idea...

9mm 124 grain bullet with a velocity of 1140 feet per second - results in a GURPS 4e damage rating of 2d6+1 based on the spreadsheet.

Were we to get its muzzle velocity to 3240 feet per second, its damage would be 7d+1. At those speeds, you may see its accuracy improve a little over that of ordinary bullet rounds.

[Fred Brackin]

Originally Posted by hal

. At those speeds, you may see its accuracy improve a little over that of ordinary bullet rounds.

Real world things maybe Gurps 4e stat ACC, no. 4e ACC is almost totally determined in 4e by gun format (pistol/rifle/etc). In 3e yes ACC was derived from range in 3e.

[Anthony]

Originally Posted by Rupert

It has to be higher than that, because modern tank guns reach that and they have massive muzzle blasts, so there's plenty of energy and pressure still in the propellant gasses.

That doesn't actually follow -- the limit is not a function of available energy, it's a function of gas velocity, so it's possible to have plenty of energy that's still in the gas but not actually available for propulsion, or that might theoretically be available but in practice is extremely inefficient to extract.