Many FMJ style military bullets have a steel jacket, to include some lots of U.S. 7.62 M80 apparently. Does this type of jacket appreciably improve the penetration of these bullets through barriers such as auto glass, walls or whatever else might be used as cover? I suspect the jacket would be quite thin so perhaps the penetration would not even match a standard FMJ, I don’t know. So… who does?
My educated guess is that differences should be minor, if any. Also there are different kinds of steel jackets, some are thinner.
We shoot IHMSA with reduced velocity loads to avoid damage (our steel silhouettes are not hard enough), and we have found that even lead bullets are very penetrative at high velocity (>2400 fps), and with lead core the type of bullet (FMJ, SP,SP, BT, nose type), sectional density, or material of the jacket (steel, copper alloy, cupronickel, etc.) does not make much difference, at least when it comes to steel velocity is king. Perhaps there is more difference in penetration of brick, cincer blocks, wood, glass, etc., I don’ know.
Now, mild steel core bullets are different, they are stonger and penetrate better than lead core ones.
No. Also, in general, most FMJ projectiles are not blind to barriers.
DocGKR, I presume you reference the standard FBI requirements when stating that they are not barrier blind. I wasn’t really expecting that they would be any sort of “Armor Piercing Lite” but I would expect more from an FMJ .308 than that. Thanks for the reply.
Well, 7.62 NATO/308 FMJ penetrates 10 mm of mild steel at 200 m… that’s something to pay attention to 
It also penetrates other covers (bricks, cincer blocks, wood, etc.) quite well. And if it fragments while passing, at least chews up the barrier with a few shots.
I don’t know about thick laminated glass, perhaps depends on the angle of impact, due to the spitzer shape of the point.
The NDIA swedish paper on their “green” (non lead core) ammo showed better penetration against concrete and brick barriers. I suppose close to mild steel core bullets.
DocGKR, does “barrier blind” Mk319 ammo actually penetrates better than M80 ball (say, thicker steel, glass, or walls)?
Or penetration is about the same but the bullet fragments and deflects less after passing certain barriers?
Or the barrier blind name is more about terminal effectiveness (“Behind barrier performance”), with or without intermediate barriers?
A projectile that is blind to barriers offers nearly identical terminal effects in both unobstructed shots, as well as those first penetrating an intermediate object, as noted here: http://www.cbd-net.com/index.php/search/show/1087257.
It has been observed that current issue 5.56x45mm munitions (M855 ball, M955 armor piercing and MK 262) are severely degraded when impacting these intervening barriers. This degradation has resulted in a diminished capability to stop the aggressive action of the threat or render him hors de combat
It’s interesting that they included AP in their list.
I guess M955 is a bit handicapped in regard to “capability to stop the aggressive action of the threat or render him hors de combat”, barrier or not 
But regarding certain barriers, M955 is obviously the way to go, even compared to “barrier blind” bullets, in spite of poor terminal behaviour (non fragmenting, non expanding, and only the hardened core penetrates). Better a hole that none at all.