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I don't understand the desire to SBR a 5.56 AR-15. The 5.56 is designed to kill with velocity, and develops markedly better ballistics with a 75-77 grain bullet at 2800-2900 FPS; that implies an 18-20" barrel. The AR-15 itself is a poor semi-auto design, not because it is globally bad, but because it trades so much away to remain controllable in full-auto. The linear action with the piston integrated within the bolt with an inline recoil buffer is exceptional, and the videos of people shooting full-auto 5.56 AR-15s with Jim Sullivan's Surefire OBC one-handed shows what the design is actually capable of.

The cost is a filthy rifle with the spring necessarily sticking straight out the back of the gun. That's a great trade if it means that you can put nearly all of the moving mass directly in line with the shooter's shoulder rather than above it, but it's very hard to take advantage of that unless you have a 3 hole receiver. In stark contrast, the AR-18 is Stoner's most popular design; the three families of service rifles that exist today are AK pattern, AR-10 pattern, and AR-18 pattern. There is a line of thought that the AR-18 was a failure, which is one of those statements that is kind of true, but only at a superficial level. The BAR was a poor light machine gun and never worked as well as its peers at its intended roles. However, the Browning's design was exceptional, and after FN was free to ignore the Army's poorly thought out requirements, they turned the BAR into the best general purpose machine gun in the world (the FN MAG/M240) and the Army's current GPMG. Not bad for a 102 year old gun. The AR-18 had a huge number of issues, mostly stemming from poor quality and a lack of buyers in the 1970s. But the fundamental idea was solid, which is why nations that develop their own service rifles use the AR-18 as as a basis over the AR-10 pattern. The UK, Japan, Germany and Singapore all developed AR-18 variants which they field today.

That's a lot of history: why? The AR-18 allows for a bullpup without the giant reciprocating mass of an AK long stroke piston. The Tavor is a tank, but it is also the antithesis of what Stoner was trying to achieve with the AR-10 in terms of keeping recoil linear with the shooter's shoulder. The short stroke piston of the AR-18 system has a relatively tiny amount of mass moving a short distance outside of the bore axis in line with the shooter's shoulder, and does not require that the recoil spring be located directly behind the bolt. If your goals are to get a powerful rifle within the shortest overall length possible, then a bullpup based on the AR-18 is very hard to beat.

To me, a gun is a barrel, a breech, a cartridge, and some means of controlling when the cartridge fires and where it goes when it does so; if part of a gun doesn't somehow add to that, it's in the same category as a balloon animal taped to the barrel. I own firearms with fixed butt stocks, but those are specific circumstances where the added benefit (aiming for a precision rifle or pointing for a shotgun) outweighs the extra length. In a full-auto AR-15, the buffer tube gives ful-auto controllability in return for longer overall length for a given barrel length. In a semi-auto AR-15, is the buffer tube giving you something in return for the length that you're adding/the barrel that you're removing to hit a length target? I can't answer that for you, but my M17S with an 18" barrel AND a Gemtech ONE is the same length as an AR-15 with no muzzle device and an 11.5" barrel (30.5" OAL).