So far as penetration through light armor, concrete, windshields and barriers? Is it really just ho-hum ammo or not? Is there any 5.56 ammo out there that is far superior that is available to the public?

M855 has the SS109 projectile with the internal steel penetrator. It penetrates very well, better than M193 or any other FMJ, SP, or JHP without a penetrator.

Some who used it against enemy personnel in Mogadishu  reported it seem to penetrate unarmored targets too well. As in leaving a small entry and exit wound with little expansion.

But M855 is M855, IMI or not.

M855A1 is pretty impressive with respect to terminal ballistics.  Penetrates very well, and has good performance in soft tissue.

Personally I think it is "ho-hum". Inconsistent terminally AND in accuracy while offering very marginal at best increase in hard cover pen. There are lots of loads that are MUCH better terminally. For bulk ammo that may possibly end up used in some SHTF scenario, I prefer plain M193.

But, I have shot metal things that the M855 penetrated while the M193 did not. So for like a Mad Max type scenario with light armored threats, it may be slightly better.

Are you talking M855 or M855A1?

I'm hardly an expert, but ive never heard about inconsistent terminal performance with A1.

As far as superior.. I’d take 77gr smk or 77gr tmk loads over any 855,  a1 included.