I wish I could remember where I read it but it was in a military reference. US Caliber .30 AP M2 was rated to penetrate 1/2" 'face-hardened steel armorplate' at 50 feet at a 90 degree angle. Armor penetration has so many variables to it that some 'standard' has to be used. Obviously most armor is sloped to increase the chance of a round bouncing off instead of ricochet. There is some formula about the angle of the armor plate and how that increases the effective thickness of the armor.
As far a personal experience, I once shot at an approximately 1 foot thick palm tree trunk and the AP went all the way through.
WWII policy of the Army was to use AP for Cal. .30 exclusivly in combat. Supply problems sometimes prevented that but "combat ammo" was AP as far as small arms was concerned.
I remember a quote from John Toland's Battle: The Story of the Bulge when a company commander was crawling around checking his men in their foxholes after a German attack. As he approached one hole he overheard two of his men, one commenting: "if we use AP, it will go right through the trees" while he was loading a clip of black tipped cartridges into the rifle.
I hope this helps and you don't mind the short history lesson. I should of signed on with a screen name of 'history nut'! :roll:
I have gotten 30-06 AP rounds from my garand to go through 3/4 inch of mild steel plate at roughly 50 yards. Hardened steel drops to just about half inch at that distance. I found another site that references the army TM 43-0001-27 "Army Ammunition Data Sheets Small Caliber Ammuntion" manual. It states "Penetration, fired at 7/8 inch thick homogenous armor plate at 100 yards, will not be less than .42 inch." I would think that an AP round fired into the engine of virtually any vehicle would do a good job of messing it up.