Armor measurement standards convertion formula

You are talking about penetration probability, which is a bit like calculating CEP, but to different standards.

The formula is the same for everyone but the standards are different.

For instance with CEP a 50% accuracy rating of 5m for a HATO round sounds impressive and the Soviet equivalent of 20m sounds terrible but they are actually the same because the HATO measure is 50% accuracy and the Soviet measure is four times higher at 95% accuracy.

Very simply with a 50% probability the CEP of 5m means if you draw a circle around the point of aim 50% of rounds fired at that target will fall somewhere within your circle... that is the HATO standard.

The Soviet standard is stricter because they demand a 95% probability of accuracy so the very same ammo on the same target the CEP would be four times bigger than the 50% level so four times 5m equals 20m... so if you draw a circle around the same point of aim that is 20m radius then 95% of all the rounds fired will be inside that circle somewhere.

Same ammo, different standards of accuracy different figures for accuracy for a given range and weapon.

Regarding ammo penetration performance instead of accuracy it was about penetration probability and is usually given as an R factor... in other words the armour penetrating 9x21mm ammo the new Russian pistols fire has an R100 of a x level body armour plate at 250m. In other words it will reliably penetrate that armour out to 250m and retain enough energy to penetrate a human body and inflict enough damage to kill the person wearing the armour.

For a 9x18mm round the R100 might be zero metres, their might be no distance where it will reliably penetrate that armour all the time, though every 5th or 6th round might penetrate within 10m its R100 would be zero.