Ok, I am going to throw in some controversy here.

AR pistols are considered by some to be suspect due to the short barrel and weaker ballistics vs a 10, 14, or 16 inch barel.

The Ammo oracle and testing suggests that heavier is better as far as ammo goes.

I know that 70+ ammo will fragment.

55 grain ammo won't.

Now, consider for a second the 5.7 pistol.

It is essentially a cut down 5.56 round, witha  40 grain projectile.

40 grains won't fragment, definately not out of a 7 inch barrel, and not out of a 4 inch barrel.

Tests I've seen suggest 9-10 inches of penetration, and the round does tumble.

Considering the 5.7 is essentially a TRUE 5.56 (+.14) pistol, here's my questions:

Is the 5.7 a wonder pistol/weapon platform (ie. P90) that provides armor penetration or a 20 shot ice pick?

Followup: In keeping with that line, if the 5.7 is a wonder pistol/weapon platform, wouldn't the AR pistol be an even better weapon platform?

Thoughts?

You mean like this?

http://www.rhinelandarms.com/

go down range, let someone shoot you with it..........report back with the results.
I dont think you'll report back........

Is that a good enough answer

Bullets don't care what cartridge launched them or what weapon shot them.

A 40gr bullet at 1600fps is still a 40gr bullet @1600fps whether from a tricked out AR or a FiveSeven.  The 40gr bullet used in the 5.7 is the Hornady V-Max, it is a rapid expanding varmint bullet, it also is not capable of defeating soft body armor out of the 5.7.    

Considering that a full power .223 55gr  is considered a poor stopper below 2500 fps, and the 5.7 can't even get a 55gr over around 1400 fps I don't see where there is any question.

The 5.7 is an excellent armor piercing handgun with the military only ammo.   The P90 is a very good subgun because of its controllability, high rate of fire, and AP capability.    The whole reason, be all and end all of the 5.7 is its AP capability.   It was designed from the ground up to replace 9mm, and provide armor piercing capability.    

The 5.7 cartridge in a U.S. semi-auto, with the civy ammo available, leaves a lot to be desired.

Its more of an ice pick.  Or small varmint gun.   Anytime the 5.7 controversy comes up, think of this:  When was the last time the .22 Hornet was proposed as a good defensive round?  and the .22 Hornet has the 5.7 beat ballistically.

Let's put it this way.....an agency that an instructor I have met shot a few people with 5.7....then went back to 5.56 so they wouldn't have to fire 17 rounds to get the bad guy down.