Agree with SKurj - the different standards operating in countries make cross-national comparisons very difficult to make. The British, for example, required 70% of shots to pass right through a target of a given thickness and quality at a particular striking angle before this thickness could be approved as its "penetration". Others used different standards.
Occasionally, some countries would put their guns side-by-side with the enemy's for a comparison test, and then the differences in standards become clear, with big differences in penetration of the same weapon and ammo being recorded.
You will also get freak incidents if you fire enough ammo at something. A .50 bullet wouldn't penetrate a 1944 Panzer tank, but it might slip in through an engine grille or aperture, knock out a track pin etc.
When fired against aircraft armour, you also need to remember that AP bullets had to work their way through the aircraft's skin and maybe structural elements before even reaching the armour, and by then they may have been knocked sideways, greatly reducing penetration.
An interesting subject I have spent some time researching, but the more you look into it, the more complicated the answers become.
Tony Williams
Military gun and ammunition website:
http://www.quarry.nildram.co.ukMilitary gun and ammunition discussion forum:
http://forums.delphiforums.com/autogun/messages/
