Guest wrote: What should be agreed on is that American and British shells were similar ( Brit better at 0 - 30 deg, Am better at oblique impact which I'm sure you all know already anyway). Yet given this the calculated penetration figures using USN Empericle formula and British test always leads to a discrepancy of 25% less for Brit armour and this after the better penetration at normal impact for Brit shells.
British and American shells were not really very similar. One of the big differences was the head shape. American head shapes were almost hemispherical. British shells were had a caliber head radius of 1.4. This is why the British shells performed better close to or at the normal, and the American shells performed better at more acute striking angles. Tests vs decks showed that the American 14" shell still out penetrated (deck penetration) the longer and heavier British 14" shell, even though the American shell was much easier to de-cap. Also the British shells usually had larger explosive cavities and thinner walls.
Differences in armour penetration has more to do with the quality of the armour rather than the penetration properties of the shells. The poor quality of American FH armour is well documented.