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Old 04-01-2017, 07:32 PM   #11
hal
 
Join Date: Aug 2004
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Default Re: GURPS ULTRATECH ballistics?

Assuming a 7.5mm caseless round, that weighs 110 grains, its velocity must be 1470 feet per second in order to inflict 2d6+2 damage.

GURPS has the ammunition weighing roughly .006 lbs per shot. This measures approximately 2.72 grams

Hmmm. That stinks!

.06 lbs per round would be 27.2 grams - subtracting 7.13 grams for the bullet, the propellent would weigh 27-7 or 20 grams.

Does 20 grams to produce 750 Joules of energy sound realistic?

That works out to 37 joules per gram? Hmm.

Just had a 'how much do you love your wife and daughter moment asking for ice cream. Time for me to bail out until later...

One thing for sure is...

2.72 grams for a 7.5mm round seems AWFULLY low. :(
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Old 04-01-2017, 07:41 PM   #12
Fred Brackin
 
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Default Re: GURPS ULTRATECH ballistics?

Quote:
Originally Posted by hal View Post
2.72 grams for a 7.5mm round seems AWFULLY low. :(
Ammo weights might be the most hashed up numbers in UT. I doubt you can do anything sensible with them.
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Old 04-01-2017, 08:03 PM   #13
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Default Re: GURPS ULTRATECH ballistics?

Quote:
Originally Posted by Fred Brackin View Post
Ammo weights might be the most hashed up numbers in UT. I doubt you can do anything sensible with them.
The rifle rounds look close to reasonable actually, but the pistol rounds are silly low weight. Apparently all UT pistol rounds are lightweight plastics or similar for some strange reason..
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Old 04-01-2017, 08:09 PM   #14
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Default Re: GURPS ULTRATECH ballistics?

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Originally Posted by DouglasCole View Post
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.
Pretty sure that doesn't work once the expansion rate is significant relative to the speed of sound in the gas; ideal gas law is for equilibrium.
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Old 04-01-2017, 08:17 PM   #15
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Default Re: GURPS ULTRATECH ballistics?

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Originally Posted by weby View Post
The rifle rounds look close to reasonable actually, but the pistol rounds are silly low weight. Apparently all UT pistol rounds are lightweight plastics or similar for some strange reason..
The issue isn't as much weight as there being clips that are lighter than the ammunition they contain.
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Old 04-01-2017, 08:37 PM   #16
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Default Re: GURPS ULTRATECH ballistics?

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Originally Posted by Anthony View Post
The issue isn't as much weight as there being clips that are lighter than the ammunition they contain.
A 10mmCLP is listed as 0.014lb total weight(The bullet and the propellant combined) that is equal to 98 grains.

Today the lightest 10mm bullets commonly used are about 135 grains, with something like 155-180 grains being more normal(and some bullets being 200 grains), then add the propellant to that and the UT total seems... very low in comparison.
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Old 04-01-2017, 08:51 PM   #17
hal
 
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Default Re: GURPS ULTRATECH ballistics?

OK, the numbers for real world damages look like they're pretty good, which means that for conventional firearms in an UT world, they should work as well.

After hitting GURPS CLASSIC MODERN FIREPOWER, the stats for the 4.73 Caseless Telescoped Ammunition indicate that it weighs .011 pounds per round, which works out to be roughly almost 5 grams in weight. Damage for the round is listed as being 4d6+2. Real world data suggests a muzzle velocity of 930 meters per second. The overall weight of the bullet is 3.25 grams. It would appear that the remaining 1.73 grams of weight is the actual propellant.

At 930 meters... that's about 3051 feet per second.

Ok, filling in those values for the spreadsheet, I end up with 3d6+2 damage (50 grains bullet weight, 3051 feet per second velocity). GURPS HIGH TECH for 4e shows that the assessed damage is still what it was in GURPS CLASSIC Modern Firepower: 4d+2.

Either Hans had his reasons for bumping up the average damage value by a full 3.5 points, or he didn't have access to this spreadsheet when he compiled the damage (or some other possibility that I've not considered).

In any event, the propellant weight of 1.73 grams will produce roughly 1401 Joules of energy. This works out to be roughly 809.8 joules of energy per gram of propellant.

This presumes of course, that the full energy available is 100% utilized, which I suspect is not the case, but it gives me a useful approximation of what should be the situation.

Now, Lead has a specific gravity of 11.34 grams per cubic centimeter? With a mass of 3.25 grams, the volume of the bullet is about .28 cubic centimeters. If the density of the propellant materials is close to that of water, then the volume of propellant is roughly 1.73 cubic centimeters to the .28 cubic centimeters of the round itself. That's roughly a 6;1 volume ratio for the propellant to projectile.

so, let's re-examine the UT 7.5mm round. Clearly, a 110 grain round is a bit much, since a 4.73 round is only 50 grains. What would be a valid bullet weight for a 7.5mm round?

Last edited by hal; 04-01-2017 at 08:54 PM. Reason: Missed the word in blue that makes all the difference
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Old 04-01-2017, 09:03 PM   #18
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Default Re: GURPS ULTRATECH ballistics?

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so, let's re-examine the UT 7.5mm round. Clearly, a 110 grain round is a bit much, since a 4.73 round is only 50 grains. What would be a valid bullet weight for a 7.5mm round?
.32 ACP tends to use 60-73 grain bullets, but something like 7.6225mm Tokarev uses typically about 90 grain bullets, so it varies.
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Old 04-01-2017, 09:36 PM   #19
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Default Re: GURPS ULTRATECH ballistics?

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Originally Posted by weby View Post
A 10mmCLP is listed as 0.014lb total weight(The bullet and the propellant combined) that is equal to 98 grains.

Today the lightest 10mm bullets commonly used are about 135 grains, with something like 155-180 grains being more normal(and some bullets being 200 grains), then add the propellant to that and the UT total seems... very low in comparison.
I'm starting to get the feeling that these damage values are Pulled out of Thin air.

Here is what I just tried to do for a pistol round of 90 grains with a 1650 feet per second statistic. The Calculated Muzzle Energy in Joules was 737 Joules and the damage was 2d6+2 per the description in ULTRATECH. On the presumption that 1 gram of modern day propellant used in the G11 round contains 809 joules of energy, I would need about .9 grams of propellant to propel a half gram bullet down range. Problem is, the original x6 volume of propellant required for the rifle round. Since this is a pistol round, the ratio of 6:1 is probably too high. Call it half that, and the ratio of 3:1 would result in roughly 1.5 grams of propellant. This in turn, would increase the joules value to a higher rating.

So, let's say that the pistol uses a 3:1 ratio for propellant volume for pistols. 1.5 Grams of propellant is 1.5 x 809 Joules or 1213.5 Joules

New round based on 3:1 ratio of propellant to projectile by volume -

90 Grain bullet
2464 feet per second
damage is 3d6+3 (average damage is 13.9 damage, which means it probably should be more like 4d+0 damage instead, or even 4d-1 damage)

bullet weight is .004 pounds per shot (it has some trailing digits after 4, but it never rounds upwards and I left it at that.). This was calculated with a bullet weight of .5 grams, and 1.5 grams of propellant to equal 2 grams per round or .004 something pounds per round). It would be safer to say that it takes 226 rounds to equal 1 pound in weight.

Does this seem MORE reasonable to you overall?

My question now becomes - what is the stopping power of the bullet in general? Seems like it is a small round, very light weight. What happens when it hits something like armor? Would it be less likely to penetrate, or more likely? THAT I couldn't answer, but it seems like there is a reason to make bullets heavier in weight for larger targets. One of the Remington rifle bullets discusses that the .30-06 government round has different bullet weights depending on whether you're varmint shooting or game shooting. 100-130 grain bullets for varmint hunting won't penetrate as well against bigger game. It is suggested 100 to 150 to 160 grain for mid-range shooting, and 180, 200, or 220 grain for big game hunting.

The flip side of this is, that we have a 3.25 gram bullet for the G11 round, and that's only 50 grain. One has to wonder if they would have found that round to be anemic against military targets or not (again, I don't know what the real life story is here, just musing in ignorance!)
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Old 04-01-2017, 09:45 PM   #20
hal
 
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Default Re: GURPS ULTRATECH ballistics?

Anyone know how to calculate the recoil of the bullets we're discussing? A .45 colt black powder weapon has a Muzzle Energy in Joules of 500 plus change. A weapon with one that is a little over twice that of a .45 colt should have a recoil of what?

Oh, I was looking at the wrong column on my spreadsheet for the ME in Joules. I was trying to get that to 1213 when I played around with the need to have 1213 Joules for 1.5 grams of propellant.

New damage is now:

3d+1 (Average damage is supposed to be 11.9) so maybe 3d+2?

Oooops, but not a major one it seems. Bullet weight remains what I figured at 2 grams, and damage is about the same, just 1 pip less.

So, anyone? How does one calculate the recoil statistic? Is it a factor of barrel length plus weapon weight or something along those lines?
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