James,
Just a note on the penetrating power of the 5.56 NATO M855 ball round on various materials. Much ink has been wasted noting the presence of a steel or tungsten “penetrator” being manufactured inside the M855 round. It weighs about 3 grains. It is insignificant so far as getting the bullet inside a car unless you open the door first. True, any load in the 5.56 will make impressive wounds or even penetrate 1/4” or even 3/8” mild steel plate…as long as there absolutely nothing in the way before it arrives on target. Put a 1/8th inch tick sheet of aluminum a foot in front of it, and witness a stunning reduction in it’s effect on the steel behind. Two sheets of 1/8” steel plate, with a foot of air between them will stop the 5.56 cold. Any load.
In my experience on several junked cars, an ordinary car door will, more often than not, stop the 5.56 before it can enter the passenger compartment and cause anything like a serious wound. Inserting a piece of 3/4” plywood inside the door of a 1988 Buick Station Wagon, I was unable to get any penetrations in the 5.56 caliber, regardless of the weight of the bullet. But note that I did not try the newer bonded LE loads, nor the ammunition using the Barnes solid copper bullets. These show better performance on auto bodies. The largest shred of bullet that even stuck to the outer veneer layer looked like a piece of glitter. Contrast this to routine through and through holes in the plywood made by garden variety 9mm, .40, and .45 pistol ammunition.
Occasionally, a bullet would hit window control hardware, or lock work, and fail to make it through, the most did. The 5.56 launches a very tiny, low mass bullet at high velocity. When it encounters any sort of layered barrier, it self destructs, yielding all of it’s energy upon whatever that material is. Heavier, sturdier .30 caliber rifle bullets represent a very serious threat to occupants of a motor vehicle, and require expensive countermeasures. But don’t be fooled by the impressive holes in homogenous steel plate, thinking the 5.56 will replicate this performance on a steel auto body or door. If you must use an AR system on a vehicle, then consider the far superior .300 AAC Blackout cartridge, launching serious high-mass .30 caliber bullets. Avoid the light weight varmint-type bullets…the 147s and 125 Sierra’s shine in this arena. – Paul S.
JWR Replies: For far more reliable penetration of car doors, .308, .30-06, and 7.62x54r will rule the day. Black tip armor piercing (AP) bullets are best, but plain old FMJ (“ball”) penetrates admirably. Yes, a .50 BMG rifle would be better, but a .308 is far more portable and versatile.