Colt did that work back in the 60's and settled on the 10.5" as the shortest practical length, the DOD adopted it in 1965 and it became the XM177. For all intents and purposes going shorter has exponentially increasing losses in power and velocity along with more muzzle blast that affects the shooter and those around them. It wasn't pleasant even then, and they adopted a moderator which became classified as a silencer by the ATF.
Noveske was a big fan of the 11.5" and touted it's many benefits. There is an incremental difference and for some it's worth it. When discussing SBR's and pistols at the 10.5" length an additional inch or loss of an inch does make some difference.
For most of us, going 10.5" means the standard length carbine buffer, whether issue or formed as a non issue pistol buffer, will result in an overall length of less than 26" from end of buffer tube to barrel muzzle. That makes it a PISTOL under the law and the benefit is that it is handled as if it were a handgun when carry and use are applied. Your state and local laws are what you would deal with and that is important. If it's over 26" it may NOT be a pistol, but some other weapon. And that means it would likely be treated as a long arm under the local law -
In my jurisdiction I can carry a pistol loaded concealed ready to hand but a long arm has to be unloaded cased in the back of the vehicle. It makes a difference.
It also goes to using a front vertical grip, under 26" you can't, over 26" you can - but it's due to that Pistol vs Other line in the sand. It's artificial but it is coded into law and it is important.
If we add the stock and it becomes an SBR, then it's a "RIFLE" and it falls under even more restrictions. Some consider owning a Stamp and having government approval as a bonus, others do not. It goes to whether you think that having a $26 accessory to improve long distance hits on a close quarters defensive weapon is worth the expense. Note that most include a red dot to improve accuracy and that is what they rely on, the stock just stabilizes the rear of the weapon and prevents any additional movement. It nonetheless still bobbles around even then, tho, doesn't it? Your choice if that makes a difference when the target is less than 21 feet away - which is the standard operational use of a short barreled firearm.
Nobody says they can't hold a 9mm and hit their target at that range, how much easier two hands and a cheek weld on a low recoiling 5.56? DI's in training demonstrate how little recoil there is by holding the butt to their chin.
You can buy and build a pistol or SBR to whatever barrel length you want, but the decision was made over 50 years ago. 10.5" is the most effective in terms of being short but still putting out bullets that can do the job. With the pistol it means using a standard carbine length buffer, which are relatively inexpensive, rather than a high cost custom short buffer system which exponentially raises the cost of the finished gun.
