Moron moves like this one are what's getting .223 ammo listed as 'Handgun Ammunition' around here.
Just what we need, more regulation on 'Handgun' ammunition.
Now, the actual 'Pistol' versions don't have a recoil tube and buffer/spring sticking out the back,
They use a much shorter tube and the bolt spring is either mounted in the upper receiver it's self or is in a housing running along the top of the barrel.
Short recoil tube version build page,
Here is a short tube link with short barrel,
Here is an example of the No Tube pistol version,
And another No Tube version,
Take something like this, which is useless in it's current configuration,
Add a shoulder sling ring and an 'Aimpoint' so you can actually pick a target, and it becomes a pretty effective close combat weapon that doesn't over penetrate.
I had my class 3 set up that way for a while, but I've gone back to a CAR stock and vertical fore grip to make it more effective in CQB situations.