It kinda depends on what you want. If you’re looking at the prevalence of rifle rated armor and thinking you might want to be able to punch through, yeah it might be worth getting some.
"Penetrating" vs "armor piercing" for 223 is there a meaningful difference?
Well there’s a
legal difference. Given the number of AR pistols out there, the ATF has decided that .223 is a “pistol cartridge” which is subject to the legal prohibition related to bullet construction for armor piercing pistol ammo. The M855 green tip penetrator was granted an exemption.
So, back to the question. 5.56 is in a bit of a weird space for armor penetration. The current armor testing standard from the NIJ 0101.06 doesn’t require a test against 5.56 so there’s the potential that some armor that meets the NIJ level III certification might be penetrated by 5.56 in some situations. Note that the new standard (0101.07) will include testing against various 5.56 rounds, but I don’t believe the standard has been officially adopted yet.
From the side of someone buying armor, you need to look for the unofficial Level III + or “special threat” rating to ensure that you’re getting protection against specific types of 5.56.
Several years ago there was a lot going around about AR500 steel plates being penetrated by M193 out of 20” barrels (trying on speed instead of bullet construction for penetration). M855 is of course moving a bit slower but has a penetrator that might come into play. Then there are poly plates that are reportedly unlikely to stop a M855 penetrator (unless specifically rated for it). Both M193 and M855 were designed to penetrate steel helmets at several hundred yards. M855 will also do better against intermediate barriers like laminated glass (windshields), cars, building materials, and other things.
Long story short, with 5.56 there is a window where at a certain range, with a certain projectile, at a certain velocity, and against a certain armor, penetration is theoretically possible. While it’s not guaranteed to always penetrate, if you’re ok giving up the terminal ballistics advantage of expanding bullets I think there’s enough of a chance of armor penetration to make it worthwhile to stock M193 or M855 as opposed to regular FMJ.