Quote Originally Posted by bombdoc View Post
You need to remember that modern 5.56 made to the SS109 standard (US Green tip etc) all have a steel pin penetrator to allow them to pass the NATO steel helmet penetration test. You "could" consider them to be AP, however I could not possibly comment..

You need to be careful in trying to estimate calibre by impact mark.. it is a bit of a black art in reality! A complicating issue is for instance the fact that the AP core is invariably smaller in diameter than the base round and is always covered by a larger jacket or sabot. Hardened steel/carbide/DU etc will not do rifling any good! The intial hole is therefor usually smaller than the original calibre. On the other hand, when a hard core strikes a plate, there are a number of ways the metal react.. it can either shatter, plug or undergo plastic deformation (..or all three!) if there is any form of plastic deformation, this will tend to expand the cavity made by the projectile, leading to a larger hole than you would expect.. It all depends on the thickness of the plate and its physical characteristics..

There are lots of types of plate armour from simple boiler plate to sophisticated layered composites. Penetration testing is done on what is called "RHA" - Rolled Homogeneous Armour, which is actually pretty soft, but is a known standard. WW2 armour tended to be case hardened cast steel, where carbon was baked into the surface to give a hard skin, but leaving a tough base. The idea was to shatter the tips of AP rounds but then capture the projectile without failing.

Firing AP or modern projectiles such as NATO 5.56 against hard targets is a seriously bad idea! What happens is that the AP pin splits the jacket from the core.. the core splatters, but the jacket is stripped off and comes back towards the firer.. which can be amusing! This is why shooting at falling plates in a gallery range is now forbidden...!
Thanks for the education on steel target/ammunition types, not something I had really thought of before, very interesting.

These steel targets 'appear' to have been hung, looking at the co-located arm.

If I had been there, I would prefer to be in the jacked up spitfire firing the 20mm cannon, behind the armoured wind screen, than stood about watching!

The noise must have been absolutely horrendous without ear defenders, not mention the danger of ricochet.