.30 cal vs intermidiate barrier

[kh86]

So 5.56 is best with expanding ammo instead of fragmenting ammo against auto glass. Is this an issue w/ 7.62x39, .308, 6.8?

[DocGKR]

Have you read this: https://www.m4carbine.net/showthread.php?t=19878

[Entropy]

It is a little more complicated than that. Here's a good read with some pictures of windshield tests:
www.dtic.mil/ndia/2008Intl/Roberts.pdf

Not all FMJ or OTM fragmenting loads are the same. Bullet construction varies between manufacturers, and it also depends materials, composition, and bullet profile. High velocity 5.56 loads tend to transfer a lot of energy to the target, but that energy is also transfered to the bullet. The light bullet of the 5.56 transfers that energy quickly to the bullet resulting in it flying to pieces when it hits a windshield. Durable bullet designs like the Fed Tac bonded can handle these stresses though and don't fly to pieces.

However, the 6.8 for example has a little more robust projectile shape with a fatter nose cone and as you can see from the photos in Dr. Roberts presentation it does well against windshields. Momentum also plays a part in overall barrier penetration.

Either way, 6.8, 7.62x39, and 7.62 NATO have much more momentum and handle windshields much better than 5.56 in FMJ or OTM.