SO, monkeying around with a 10.5" 5.56 melonited barrel to see what kind of accuracy I could get from it. Worked some 77 SMK over 8208 XBR and managed roughly 1.25-1.5 5 shot grouping at 100 with a scope. Has anyone worked with a match ss barrel in short length. Kind of a "mini SPR"?

There's the big mini-Recce rifle thread.

http://www.ar15.com/forums/t_3_118/659371_Official-MINI-RECCE-Photo-and-Discussion-Thread.html

Guess that may be a start. Looks like another project

Everything's a compromise depending on exactly what one is trying to achieve with a given caliber or weapon setup.  However, over a period of time reading and seeing reports from quite a few respected AR so-called gurus, I've come to the conclusion that 11.5" can be argued to provide the best barrel length for velocity, accuracy, and range among the many options available.  I tested this with my 14.5", 10.5", and 11.5" setups, and I believe I agree.  I reload also, so I've been able to even play with bullet and powder combinations as applied to barrel length.  I've pretty much settled on the 11.5" setup as the dust has settled.  Both my 10.5 and 11.5 barrels are from the same manufacturer with the same 1:7 twist by the way.

Agree, the 11.5" seems to have advantages over the 10.5. Although looking at various charts, even in other calibers, the 12.5 always seems to be the sweet spot.

No doubt that the longer barrel almost always produces higher velocity and therefore the potential for better accuracy.  However, we're talking keeping the gun within the pistol format, as you leave the 10.5 and 11.5 area you're really getting close to the 14.5 rifle area...with fixed muzzle device of course...and there's generally some magic at the 14.5" barrel length with 5.56.  This is where the 12.5" becomes too much of a compromise as it relates to a pistol format IMO.  If you're going for the shortest rifle, SBR, format it starts to make a little more sense, but for 2" more you gain a very noticeable performance increase with the 14.5" without any NFA heartburn as long as you have a fixed muzzle device...and face it, you're going to have some kind of muzzle device on the pistol lengths too in real world use.

Here's another curve ball to the 11.5" vs 12.5" barrel.  If you notice the better tests performed on barrel length tend to show less velocity difference between the two as bullet weight increases.  This is why I shoot my own reloads in the 68g bullet.  I'll also contend that the shorter barrel likes the heavier bullet in general as long as you stay in the 1:7/1:8 envelope.  If one is going to only shoot 55g bulk ammo...well...then it appears that the 11.5 loses anywhere from about 30-90 fps depending on ammo.  Still, I think you start getting away from why you wanted to get into a pistol format in the first place.

The key word is "compromise".  Throw in a good dose of preference, and the whole decision process gets even fuzzier.  This may be one reason why lots of AR guys have several pistols in 5.56.

Indeed, one my first pistol, wanted it to be that and did the 10.5 in 5.56. The .300's are 8/9.