Does green tip (ss109 or M855) penetrate body armor out of pistol-length barrels?

Here's some new data.

Fired at a range of just a few yards, M855 will not penetrate an AR500 Brinnell Hardness 500 personal armor steel plate. The M193 will. NATO adopted the M855, but not the M193, because the M855 was considered more humane. See the video at https://www.youtube.com/watch?v=oMYkEMhPsO8 and the reference at Wikipedia.

Since M855 will not penetrate a Level III steel plate when fired from a long barrel, it follows that it will also not penetrate a lower speed.

As an addendum to denton's excellent post, I'll point out that according to the narrator in that video, M193 will only penetrate an AR500 plate at extremely close range from a 20" or longer barrel, and will not penetrate at any range from a 16" or shorter barrel. M855 did not penetrate even when fired from a 22" barrel.

There is another video in which a guy shoots an AR500 plate 90 times at 35 yards with M855 out of what looks like an 18" barrel. There were no penetrations at all prior to 60 rounds, despite multiple rounds hitting in the same dimples, until finally the plate got so fatigued/eroded that two rounds finally slipped through some time between impact #60 and impact #90.

http://www.youtube.com/watch?v=d2TBoP9Aoow

If a NIJ Level III plate will withstand 60 hits from M855 out of an 18" AR at 35 yards, then it will withstand M855 at any range from a pistol length barrel, period. M855 is going only 1900 ft/sec or so out of a 7" barrel and maybe 2400-2650 ft/sec out of an 11.5", if I have my numbers right. In the video denton posted, a III-rated plate stopped a round of M855 going 3000+ ft/sec.

Having said that, I will point out that this is NIJ Level III hard armor here, not IIIA. NIJ IIIA will not stop any rifle round, although it is possible that M855 out of some super-short barrels might be stopped by some vests (since M855 out of a 7" barrel is going 1900 ft/sec or less).

NIJ IIIA will not stop any rifle round

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I absolutely believe you, but could you give us the basis for this statement?

The problem is comparing apples and oranges.
The apples is AR500 steel plate, real armor steel.
The oranges is vests worn by people that are a compromise because nobody is willing to wear a 70 pound suit that is actual armor. Vests are far better than nothing and effective against handgun rounds. But they are NOT "armor" as in AR500 steel type.

The National Institutes of Justice armor ratings are set forth in Ballistic Resistance of Body Armor - NIJ Standard-0101.06. The specification for Level IIIA soft armor is as follows:

Type IIIA (.357 SIG; .44 Magnum)
Type IIIA armor that is new and unworn shall be tested with .357 SIG FMJ Flat Nose
(FN) bullets with a specified mass of 8.1 g (125 gr) and a velocity of 448 m/s ± 9.1 m/s (1470ft/s ± 30 ft/s) and with .44 Magnum Semi Jacketed Hollow Point (SJHP) bullets with a specified mass of 15.6 g (240 gr) and a velocity of 436 m/s ± 9.1 m/s (1430 ft/s ± 30 ft/s).

Type IIIA armor that has been conditioned shall be tested with .357 SIG FMJ FN bullets
with a specified mass of 8.1 g (125 gr) and a velocity of 430 m/s ± 9.1 m/s (1410 ft/s ± 30 ft/s) and with .44 Magnum SJHP bullets with a specified mass of 15.6 g (240 gr) and a velocity of 408 m/s ± 9.1 m/s (1340 ft/s ± 30 ft/s).

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So it is rated to stop a 125-grain 0.355" bullet at 1470 ft/sec, or a 240-grain 0.429" bullet at 1430 ft/sec, both of fairly low sectional density. IIIA is known to stop some bullets that are smaller and faster (e.g. commercial 5.7x28mm out of a pistol length barrel), but not that much faster due to the physical limits of Kevlar. Almost any centerfire rifle cartridge designed since 1894 will exceed those parameters by a very wide margin.

Pistol rounds are relatively low-pressure as firearms cartridges go, and they don't typically have much barrel length to make the best use of what pressure they have to work with, so pistol (and most revolver) rounds tend to be relatively slow (Mach 1.4 or less) and low energy (relatively few handgun rounds reach even 600 ft-lb of energy at the muzzle). As a result, you can stop a handgun round by catching it in a mesh of lightweight, strong polymer fibers that stretch and break as they decelerate the bullet within a couple inches or so. Rifle rounds have too much energy for that; even the lowly .223 Remington exceeds 1200 ft-lb and 2800 ft/sec out of a 16" barrel, and most deer rifles carry between 2000-3000 ft-lb, and you can't decelerate a projectile with that much velocity and energy with any known fibers within the space of a couple inches. (If you could let the net deform a foot or two, it'd be easy, but a vest can't deform that much without grievously injuring the wearer.) Instead, you have to cause the bullet to expend its energy disintegrating itself against something harder than itself (e.g. Level III rated AR500 plate), or dissipate its energy crushing and pulverizing an extremely hard ceramic (Level IV plate, the only practical way to stop tungsten or tungsten carbide core AP rounds).

The gist of the armor ratings are as follows:

NIJ Level IIA (soft armor) - rated to stop slow 9mm, .40 Smith & Wesson (handgun rounds)
NIJ Level II (soft armor) - rated to stop fast 9mm, slow .357 (handgun rounds)
NIJ Level IIIA (soft armor) - rated to stop .357 Sig, .44 mag (handgun rounds)
NIJ Level III (hard armor) - rated to stop steel jacketed 147gr 7.62x51mm at 2780 ft/sec (substantial rifle round)
NIJ Level IV (hard armor) - rated to stop full power rifle shooting armor piercing ammunition (166gr .30-06/7.62x63mm tungsten-core AP at 2880 ft/sec)

For context, M855 is a 62-grain, copper jacketed, steel-capped lead core .22-caliber bullet at about 2980 ft/sec out of a 20" barrel.

Great explanation. Thanks.