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I shoot the Hippopotamus
With bullets made of platinum,
Because if I use leaden ones
His hide is sure to flatten 'em.
-- Hilaire Belloc

Data and Background

     First, data gathering page and discussion. Source info:

• .30-06 ball will perforate a 0.4" (9.5mm) mild steel pate at 100yds, 0.3" (7.6mm) at 200 yards, and 0.1" (2.5mm) at 600 yards.

• The 1939 US Army specification for M-2 .30-06 AP penetration is .42" (11mm) of armor steel at 100 feet.

• 7.92mm German ball ammunition penetrates 0.2" (5mm) of steel (type not specified) at 100 yards, 0.1" (3mm) at 600 yards.

• 7.92mm German AP (steel core) penetrates (0.47") 12mm of armor steel at 100 yards. Developed specifically to deal with Great War tanks, German infantrymen in 1918 were being issued 10 rounds each of this if a tank came along. Even before the end of the Great War, this round became an ineffective anti-tank weapon.

• 7.92mm German AP (tungsten core) penetrates 0.79" (20mm) in mild steel at 550 yards. This tungsten-core ammunition was introduced just before WW2, but was never more than 2% of production, and was discontinued in 1942.

• 7.62mm NATO AP M993 rounds will penetrate 16mm of RHA steel at 300 meters range. This is a modern tungsten-core round.
  

• 5.56mm NATO AP M995 rounds will penetrate 12mm of RHA steel at 100 meters range. This is a modern tungsten-core round. 

• 7.5mm Mle 1929 AP rounds will penetrate 8mm of RHA steel at 50 meters range.

• 7.65mm Mauser FMJ pistol rounds (aka .30 Mauser) will penetrate 3mm of mild steel at 20 meters range, or 41 cm of solid pine at 30 m range
  

  In general, I'd say rifle-caliber ball ammunition will be defeated by 8mm of armor steel at any range. Armor-piercing (steel core) rifle-caliber ammunition will be defeated by 12mm of armor steel at any range. Tungsten-core AP rounds will be defeated by (my estimate) 16mm of armor steel at any range.

     Mild (commercial) steel used as armor needs to be about 25% thicker to have equal effect ... so 10mm, 15mm and 20mm respectively (vs. ball, steel AP, or tungsten AP).

     There's a reason you see a lot of armored cars and very light tanks with about 8mm armor in this period ... and remember, if the armor is angled at all, and as the range increases, the chance of penetration will decrease. Armor sloped at 60 degrees is equivalent to about twice the thickness (for projectiles travelling more or less horizontally); so those nicely-geometric German armored cars probably have about 12mm of armor equivalent (based on an actual thickness of 8mm).

     Note that performance of armor-piercing rifle ammunition has increased a lot since the 1930s: better materials, and designed to be less affected by sloped armor. As steel gets above an inch or two thick, or as the type of steel changes -- for example, to face-hardened steel -- this chart will be less accurate.

     The following table will allow us to compile and compare real-life information about period armor penetration.

printed sources:

• "FF" is World War 2 Fact Files: Anti-Tank Weapons, by Peter & Terry Gander, pub. MacDonald & Jane's, 1974
  

• "MSA" is Military Small Arms of the 20th Century, by Ian V. Hogg and John Weeks, pub. (6th 3d) DBI Books
  

• "BM" is A Basic Military Manual of Small Arms, by W.H.B. Smith, third edition, pub. October 1945

• "TCA" is Twentieth-Century Artillery, by Ian V. Hogg, pub. Amber Books, 2000

• "RPG" is The Rocket-Propelled Grenade, by Gordon Rottman, pub. Osprey Publishing, 2010
  

   

online sources with reasonably primary information:

• Tactical and Technical Trends page, info taken from a British 1944 report, "TTT"
  

• German 88mm Panzerschrek page, "G88"

• WW2equipment.com page, "WW2"

• French Tank and Armoured Vehicles Armament page

   

Armor Materials

     Use this chart to determine the equivalent thickness of RHA steel:

     RHA stands for "rolled homogenous armor"; HHA stands for "high hardness armor". Note that these figures refer to 1930s materials; concrete in particular has been subject to much improvement in strength since then. 1/32" is about 22 gauge steel, by the way.

Example:  a 6" thick wall of ordinary concrete. That's 152mm, multiplied by 0.11, gives the equivalent thickness of RHA steel as 16.8mm.

     Don't expect this to be accurate for everything from rifle bullets to battleship shells. A normal sandbag is about 12" wide when filled; if filled with dry sand, it's equivalent to about 11mm of armor steel. Thus at anything except short range it'll keep out regular ("ball") and normal armor-piercing rifle bullets.

     A standard cemented brick wall for a two-story structure is 8" thick:  the same as 1.2" of armor steel.

Armor Conversion

     Using the RHA equivalent value from the previous sectioni, find the Armor Value in the right-hand column below.

     For values between two listed Armor Values, use the lower value (up to about Armor Value 30). Don't add Armor Values together -- it's not a linear scale. Instead, determine the RHA equivalent for the materials, add those together, and use the Armor Value for the total. For layered fabric armor, it's best to use known examples.

Example:  16.8mm (2/3 of an inch) of RHA steel has an armor value of 20.

Penetration by Explosives

     The table entries are for pounds of the listed explosive (except of course the last two rows).

     Perforation presumes untamped contact. Tamped explosives will go through more armor (let's say 3 more points); well-placed tamped explosives even more (say 5 points more than just "untamped in contact"). Explosives out of contact will go through 3 points less armor, before even taking the actual distance into account. 

Defining Ammunition Performance

     Subtracting a "piercing" value set for each ammunition type seems to be the best combination of simple and somewhat realistic. For rifles, subtract 3 points from the armor for armor-piercing bullets; subtract 5 points for tungsten-core armor-piercing bullets. The Fury gun 45mm shaped charge would subtract about 10 points from armor in the current "Mark 0" version; an improved version, capable of penetrating about 2" of armor steel, would have 18 points of "piercing" (I doubt the actual damage would increase, since the amount of explosive would remain the same). There will not be any correlation between the "piercing" number and projectile size and construction, since the number is an adjustment to the VERY arbitrary damage dice.

Weapon Penetration

     Pistol ammunition, and regular ("ball") rifle ammunition, just roll their regular damage. Armor-piercing rifle bullets should subtract 3 points from the armor; tungsten-core armor-piercing bullets should subtract 5 points from the armor. Other heavy weapons (including anti-tank rifles) will have a "piercing" number, to be subtracted from the defenses.

Example:  the USS Constitution has sides of 21" thick oak. The conversion for oak to armor steel is 0.012, this is equivalent to 0.25" of armor steel, or about 14 points of Call of Cthulhu armor. Thus a cannonball just able to penetrate the hull half the time would do 4d6 damage, and modern rifle bullets are mostly kept out -- and in fact a .30-06 bullet will go through 27" of oak.

Example:  a 15th Century European breastplate is 2mm thick, of very mild steel; equivalent to 1.2mm of Thirties rolled homogenous armor steel. It should provide about 7 points of armor. I'm using a figure halfway between wrought iron and 1930s mild steel, by the way. Eh, the Call of Cthulhu system doesn't really support all the reasons that people don't wear plate armor any more.

"On the morning after Quatre Bras, some British soldiers looked for something to cook their breakfast in and found the breastplates of the dead cuirassiers very useful. The only drawback was that much of the meat's juices were lost through the bullet holes in the breast plate."

Example:   a "siege weight" 16th Century helmet is 3mm of very mild steel, equivalent to 1.8mm of RHA steel; it provides 8 points of armor, maybe 9 if you expect there were advances in metallurgy between the 15th and 16th centuries.

Example:  the Brodie helmet, aka the doughboy helmet or Mark I helmet, is made of 0.9mm thick armor steel. The German Stahlheim helmet has a thickness of 1.1mm armor steel. Both should provide about 5 points of armor. The Call of Cthulhu rules give the armor of a World War 1 helmet as 2 points, hmm. Would you care to try punching or kicking a hole in my Brodie helmet?

Example:  A normal sandbag is about 12" wide when filled; if filled with dry sand, it's equivalent to about 10.7mm of armor steel, or 17 points of armor.