pathfinder74
Let's see if we can hit on a few things you are asking about... First as to the accuracy potential of a firearm and the length of the barrel -- in all honesty, most of what a barrel does, is done withing the first few inches, the stability and spin are well established within inches -- if you think about it, with a 1:7 twist barrel, the bullet has already been spun one full turn at 7" and will certainly not benfit from anymore than that.
The only thing that is gained after the first half a foot or so is velocity... the military testing concluded that 18" is the shortest a barrel can be cut and still maintain *near* maximum potential for velocity. This is based on the efficency of the barrel to allow the charge to burn, under 18" the loss is exponential and performace degrades quickly... The M4, at 14.5 is a trade off of handling and length (important in the current urban and vehicle borne fighting), but many say it is as short as one would ever want to go for a rifle/carbine.
The CQB type weapons are naturally shorter, but keep in mind that we are dealing with confines that place people well withing what we like to call "the range of normal social intercourse" -- Sure, they work a football field away, but that is not what they are designed for and certainly second place in that catagory... again, it is a trade off, the lessor of two evils. When you are in a space that you literally can not let another person get past you, you have to make concessions for certain things.
Reliability of the short barreled guns is pretty good at about a foot of barrel or a little less, but below that is starts to suck as bad as MTV... Pressures and timing are way out of whack and with the port located closer than about 7" (for the 5.56/223) we are not even really talking gas at this point, the efficiency of a barrel this short is a joke and the life of a suppressor would be drastically short, not to mention that you would have to use a pretty good size can to make the weapon tolerable at all -- a lot of people fail to realize this, they want a 10 inch barrel, but have to hang 7" of suppressor off the front to tame it down, when they could have went with a 14.5 and a much smaller can to get a lighter AND shorter setup than the 10" barrel.
The deal is that the 5.56/223 was not built to use in uber quiet, room to room stalks... it can be made acceptable, but not what it was made for -- I have a 300 Whisper that is really nice for this, as well as a couple of other toys my Uncle uses... one of them, all you can hear is the firing pin snap! Marty at Tepo Jutsu sent me a 338 Spectre barrel recently, if I ever get enough time to get it set up, I am sure this will be a very quite little rig too!
There is no way to compare a pistol caliber carbine to the 223, this is apples and handgrenades... but I would suggest, if you want what it sounds like you want, to talk to JD Jones, Steve at ADCO, Randall at ar15barrels or Marty at TJ about getting a short, suppressed gun in a round that is designed for that purpose... the Whipser is balls accurate and hits harder at 100 yards than a 45 ACP does at the muzzle...