Looking for Stats for 90mm and 120mm tank guns?

[Phantasm]

My search-fu is incredibly weak right now, because I know this has been asked before.

Basically, I'm looking to see if anyone's come up with stats for the current 90mm and 120mm tank guns presently fielded by western nations on their LAVs and MBTs. I'm particularly interested in the damages of HEAT and APFSDSDU rounds fired and their effective ranges.

[Litvyak]

If you can find real world stats for armor penetration in mm for a particular shell, then you simply multiply that number by 0.7874 to get GURPS dice of damage (this number comes from an inch of armor steel being worth 70 DR, divided by 25.4 to convert to mm, and then divided by 3.5 to convert to dice).

[hal]

Originally Posted by Litvyak

If you can find real world stats for armor penetration in mm for a particular shell, then you simply multiply that number by 0.7874 to get GURPS dice of damage (this number comes from an inch of armor steel being worth 70 DR, divided by 25.4 to convert to mm, and then divided by 3.5 to convert to dice).

Test that approach on bullets to see if the theory works. Otherwise, you may want to use the ballistics formula floating out there

[Phantasm]

Originally Posted by hal

Test that approach on bullets to see if the theory works. Otherwise, you may want to use the ballistics formula floating out there

I've heard it exists, but I've never actually seen the formula listed.

I'm especially curious about how stuff that's obviously designed to be armor-piercing (HEAT with its AD (10), APDS and its derivatives with its AD (2), etc.) is handled by this formula.

[RyanW]

Originally Posted by Litvyak

If you can find real world stats for armor penetration in mm for a particular shell, then you simply multiply that number by 0.7874 to get GURPS dice of damage (this number comes from an inch of armor steel being worth 70 DR, divided by 25.4 to convert to mm, and then divided by 3.5 to convert to dice).

They would be firing some version of AP (current is APFSDS or APFSDSDU) so you would also have to account for their armor divisors and any other adjustments, which would be listed in High Tech.

[RyanW]

Originally Posted by Phantasm

I've heard it exists, but I've never actually seen the formula listed.

There are two formulas. Douglas Cole has reverse engineered one from the available stats, and then there is the official unpublished one. I believe they tend to diverge the further you get away from small arms.

[mlangsdorf]

The Russion 2A46M 125mm smoothbore gun, used on the T-72 and later tanks, is in High-Tech pp 140-141 as the Motovilikha D-81TM. The table stats are for the HEAT round, but APFSDS and APFSDSDU stats are given in the write-up.

I can't quickly find any penetration numbers for the 2A46M that I can trust, so I can't extrapolate the NATO M256 120mm main gun's numbers from the 2A46M.

Wikipedia says the M829A1 APFSDSDU round penetrates around 24" of RHA at 1000 yards, which is 6dx40 (2). That's ~25% more penetration than the 2A46M, despite using a less massive dart that is probably going somewhat slower.* I think something in the 6dx30 to the 6dx40 range is probably right.

* Accurate numbers on modern tank guns are frustratingly hard to get. Per wikipedia, the 2A46 fires a 15 lb dart at around 5400 ft/s with a 12 lb charge down a 48 caliber barrel. The M256 fires a 10 lb dart at 5170 ft/s with a 17 lb charge down a 44 caliber barrel. Either NATO propellant is terrible or some of those numbers are wrong. But wikipedia also claims that the 2A46M can barely penetrate 350mm of RHA, and that's pretty low for a 120+mm gun.

[mlangsdorf]

Originally Posted by Phantasm

I've heard it exists, but I've never actually seen the formula listed.

Average damage, in points, is 1/13.3926 * square root (KE^1.04 / projectile cross sectional area ^0.314). Divide by 3.5 to get damage in dice or by 21 to get damage in 6x dice. KE is in Joules and cross sectional area is in meters^2. Or at least that's the version I copied from one of Doug's spreadsheets.

It's good to about +/-5% for small arms, depending on exactly what bullet loading and barrel length you consider typical for a weapon. It diverges a bit more for heavier weapons, up to about 20%, but I think some of that comes from my using different numbers for velocity and shell weight than the GURPS authors.

[hal]

Originally Posted by mlangsdorf

Average damage, in points, is 1/13.3926 * square root (KE^1.04 / projectile cross sectional area ^0.314). Divide by 3.5 to get damage in dice or by 21 to get damage in 6x dice. KE is in Joules and cross sectional area is in meters^2. Or at least that's the version I copied from one of Doug's spreadsheets.

It's good to about +/-5% for small arms, depending on exactly what bullet loading and barrel length you consider typical for a weapon. It diverges a bit more for heavier weapons, up to about 20%, but I think some of that comes from my using different numbers for velocity and shell weight than the GURPS authors.

Some of the other issues arise with respect to where and when the ballistics information was gathered. Even the temperature can change the ballistics of a projectile somewhat. Then, looking at Cartridges of the World - it seems that not all bullets are the same with respect to weights - so which bullet was chosen as representative over others can likely be an issue.

The +/- 20% is significant to be sure, but a 5% difference probably won't matter all too much one way or another.

As it was? A discussion I had with another about "accuracy" values for the Walther PPK made him object strenuously saying "It can't be THAT inaccurate as to rate a zero accuracy if it had been issued in large numbers for Government use!" I let him know that the accuracy values in GURPS are VERY small and have a small range (0 to 2 for pistols for the most part, and 3 for black powder rifles, and generally speaking 4+ (just glancing at the tables quickly that is) with 6 seemingly the highest accuracy for modern rifles (disallowing for the bonus for optics). How Cannon barrels rate I've not looked at, so I haven't the opinion (which is often ill-informed anyhow!).

While Doug's spreadsheet seems to be more metric oriented, the one I worked on FROM Doug's spreadsheet, will generate the bullet damage rating along with Half Damage range and Max damage range (using Doug's formulas) but instead of the route he went to get his numbers, I simply fed in Bullet weight, Diameter, and bullet velocity to generate my numbers. It seems to work, and eventually, I will undertake the task of creating a database in Excel, of all the weapons listed in THE GREENHILL MILITARY SMALL ARMS DATA BOOK (weapons from all nationalities, current up to about 1999 when the book was published). I also compiled a database of black powder weapons from Cartridges of the World as well as another Reference book on the weapons of the west. When I compared the values given in GURPS HIGH TECH ADVENTURE GUNS, they were largely the same (with a few that were off by 3d-1 versus 2d+2 type of thing).

In any event, GUNS GUNS GUNS Or 3G3 as it is now called, has formulas for calculating damage values for various weapons and even has a conversion formula for translating the 3G3 Damage Values into various other game systems.

When SJGames is hampered by not being able to print all of the GURPS reference works we'd like to have, we just have to go out and make them up for our own campaigns (where possible). I have two Tank data books on Captured German Tanks as well as American Tanks used in World War TWO that could be useful for determining DR of hulls (armor is listed by thickness in various locations), as well as speeds, weight, etc. I don't recall if they included "gun" statistics in the book or not.

[HANS]

Originally Posted by hal

As it was? A discussion I had with another about "accuracy" values for the Walther PPK made him object strenuously saying "It can't be THAT inaccurate as to rate a zero accuracy if it had been issued in large numbers for Government use!"

And it doesn't. The Walther PPK has Acc 1 (High-Tech, p. 101). Which is entirely justified given that almost all larger handguns with longer barrels and considerably longer sightlines have Acc 2. Also, the "government use" of the PPK was confined entirely to it being issued as a self-defence weapon for close quarters, for pilots, plain clothes detectives, etc.

Cheers

HANS