They use the Knights Armaments M110 here is a link, there are two versions. SASS is what the program was called to adopt a rifle (even though everyone new knights was already chosen). These rifles were not envisioned to provide precision fires past 800M. They were chosen for rapid engagements and compact handling in urban environments.
The standard match round for the M110/M14 is the LR118 round, It fires a 175g SMK behind R15 in LC brass (I believe they have now switch powders). The barrel length is not as key in the 308 as the twist rate and the bullet fired. I believe the general rule of thumb is 50 fps for every inch of barrel how ever the 308 is an exception.
The other precision fires platforms with in the military the M24 and M40 use a 24" barrel. However there was a program fielded a few years back by the marines in which 20" barreled versions were use with overwhelming success. The Army has chose to upgrade there M24 to 300 win mag and use the M110 for closer stuff.
If you want to use the search function over at M4c "KevinB" "sinister" are names to look for. Kevin is military sales rep for knights and sinister is a seal, with lots of hands on experience with these platforms. If you decide to join the forum be warned that they are not fond of certain companies and will openly ridicule anyone.
Its also important to note at this point that your rifle is not a bolt gun and has a floating firing pin. Whats this mean? when you close the bolt the firing pin actually impacts the primer, this can cause a out of battery detonation of your round destroying your weapon and maybe you. The military use a primer with a thicker cup to account for this (CCI #41). Next time you chamber a round extract it and look at the primer you will see a dent ,scary! Additionally as I mentioned in the beginning, your rifle is not a bolt gun. This means you have to be super careful with ammo selection NO SUPER PERFORMANCE AMMO, and when you reloading you need to reload for a gas gun that means not all powders are OK to use in your rifle just because they are in the manual for 308 Winchester.
God its so much info and I did not come by it all over night and this is by no means all of it, but I hope I'm helping some. Also I like short barrels 16" will get it done with the right bullet, I would never go longer than a 20". If you swap and find out you want a shorter one you can have it cut down, but then you will also need your gas port corrected or your rifle will not work there is also the possibility in that case you will need to swap buffer or spring, this is just FYI info.
Jon
Military | Weapons | Accessories | Knight's Armament Company
Also look through this and see if you can get any ideas.
Precision Rifle - Semi-Auto PICTURE THREAD - M4Carbine.net Forums