I know 5.45x39 is not on the testing list for Level III and IV plates, but since it is a popular cartridge in the US and abroad I wondered if anyone has tested it?  From the ballistics tables it looks as if it would be comparable to a slower .223, but I was interested in any real-world testing.

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I know level III AR500 steel plates have been tested against 5.45 extensively. If  LVIII steel can stop it ceramic should be able to no problem.

not true
Distance is the key here.
I have 55gn reloads and 5.45 mil spec that eat ar 500 seel and texture covered ar 500 steel from the "big seller" at and inside 25 yd.  
Eat as in punch a clean hole through the plate removing a solid piece, not a lucky punch through or partial pass.
Sapis do well to stop 7.62 rounds at close range Esapis stop 5.56 and 5.45 and 7.62 ap at close range as shown
Steel will not stop 5.56 and 5.45 inside of 25 yds unless you are referencing a thicker product I have not seen commonly marketed.
For those interested in an odd bit of history the weight and velocity proved it relevancy in the LE MAS ammo debacle.  A light gn varmint round would zip right through steel, due to the increased velocity.
This is why inside a certain range 55 gn penetrates better than 62 gn.

My unlined AR500 steel armor plates (that I use for target shooting, not armor), will stop 5.45x39 no problem within 25 yds.

What brand ammo?
And when you say plates for target shooting, are they armor plates or ar 500 targets, there is a difference.
Thanks
My ar500 targets are significantly thicker and stop fast rounds close as well, but decrease the service life of the target and do pock and damage it when not mounted to allow reciprocal movement (like on a t post).
Steel will stop wolf, Barnaul, etc inside 25 yds in both 5.56/223 and 5.45 due to a lower velocity.
I will clarify that 7n6 and (x)m193 are adequate for defeating most non enhanced sapi plates and steel inside of 25 yds.  
The rating system (nij) and military plate ratings exist for a reason, a low cost alternative might work, but why take a chance with an un tested non serialized plate?

Target plates are usually 3/8 inch thick, body armor is 1/4 inch.  Big difference

Thanks for the video link.  I'm not surprised at the results, but it is good to have some confirmation.

AR500 level 3 steel armor plates have been proven to very reliably stop 5.45 7n6 steel core from 20 yards - not sure how you're punching through whatever it is you have with 5.45.

When you say "AR500 will not stop 5.56 inside of 25 yards" you are also making a partially untrue blanket statement. AR500 won't stop M193 55 grain rounds shot out of a 16" or greater barreled rifle at 25 yards or closer. It WILL however stop M855 and in general heavier grain, slower rounds.

Video proof