With all due respect Mr. Williams your data is not quite up to date, or the most widely accepted as being accurate this day in age. I too am in LE with over 20 yrs. I have been a long range / precision shooter and in the sniper community for quite some time. I am currently a sniper and a part of the Training Cadre for a full time SWAT unit for a major metro PD. I am the lead on the firearms side of the house. Ballistics, yada, yada, is a part of my bread and butter so to speak, besides mulling over data, I actually test everything that I can think of. Literally. This includes testing the short barrel theory by cutting in increments and shooting hundreds of rounds in the process, logging data.
Having said that my group of guys also shoot the above mentioned rifle, in the TacOps offerings. As a LE unit we probably field more of these rifles than just about any one group and for a very good reason. We have 19" barrels and primarily run suppressed and we do use the Federal 168gr GMM ammo.
While I am an “old school” or “old dog” type, I am at the same time very progressive in my methodology. I do not resist change or new ideas, just because it has worked for me for the last 20 years. I don’t change “just because”, as I only make true changes to my methodology only after vigorous testing on my own accord and finding out that something is truly better. We need to be able to be open minded enough to progress forward on tactics and technology. Being stagnant IMO is not a good thing.
Bullets improve, powder improves, ballistics improve, materials improve, weapons improve. All of this leads us to have to re-think or adapt our “old school” attitudes or techniques to progress forward. If not, we would still be throwing rocks and using clubs.
Regarding .308, as mentioned, FGMM is completely burnt in about 19" of barrel. Black Hills match ammo is good in 18". Not saying that you can’t go less barrel and still not have a great performing rifle. Again accuracy is not as heavily related to velocity as one might wish to believe. If we look at the normal effective range of the .308 at around 800y-1000y or even go beyond those numbers say out to 1200y we really see no quantifiable affect on accuracy, especially one that we can purely relate to the rifle and ammo itself.
Do we have a loss in velocity? Yes, there is, which can be measured by chronograph or perhaps more importantly on a range by actually shooting. Now how much velocity do we lose and how does that affect the actual performance of the rifle? Well let me just sum this one up, with actual shooting sessions and logging of hundreds and hundreds of rounds. I will say that going from a 26" barrel shooting the 168 gr FGMM ammo and the Black Hills 175gr Match, we see no noticeable affect in bullet performance in regards to the velocity numbers at up to 550 yards. In other words at any barrel length from 26" to 18" with above ammo we do not dial in any any change to our DOPE up to 550 yards. :eek: Shocking, but true. I never believed it the first time I did this and it took some time to sink in. I would have bet just about anything that this was not possible. However this is not just the norm for one rifle, but every single rifle that I have seen, in this length range using the above factory match type ammo, and that is more rifles than I can count off the top of my head.
When we work up to distances nearing 1000y we see on average an increase of 2 additional mins correction in the Black Hills and around 4 mins increase in the FGMM. Again I was amazed by this, but again it holds true in all rifles that I have seen. All of this shows no signs in a lack of accuracy. So in essence even at these distances there is no ballistic effect on the .308 round in regards to a loss in velocity that we can reasonably claim a decrease in accuracy. For myself, out past 800 I tend to take into account spin drift voodoo and dial in some windage, but that is another topic.
I have yet to see any rifle cut down decrease in accuracy from a 100y zero. In reality, the more common effect is increased accuracy. Perhaps Mike R. from TacOps had this figured out a long time before it became mainstream and he has been building his rifles accordingly for many years now. Arguably the best LE tactical rifle ever created. The best I have ever handled and I have shot more than my fair share.
I know you have talked about long range precision shooters, but the needs there may be much different than a tactical shooter. For competition, having a heavy pig of a rifle is often a plus. Helps keep the weapon steady for overly magnified optics, absorbs felt recoil and also helps to keep your reticule on target. I know this because I have done my share of benchrest types of shooting also. I have even shot my 19" barrel against some top rated shooters and their custom weapons in the “for fun” shooting community. Some are enlightened but most get upset, scoff and think it a fluke. Which is always entertaining.
Now when I get into the weight savings topic, which dramatically improves ease of stalking, maneuverability, quickness of transitioning to multiple targets, tracking moving targets, increased shooter accuracy especially when positional shooting, you name it, the list goes on, there is no reasonable downside that I have been able to come up with to make me not wish to go the shorter barrel route. The noise, muzzle blast, ground disturbance etc argument is not one that I give much merit because if you are truly concerned about those issues and your need dictates that you mitigate these issues, than you should be running suppressed no matter the barrel length. And I will say that if you are going to run a suppressor on a tactical rifle IMO you had best be running a shorter barrel to begin with. 
In summary, theoretically certain things may not add up on paper. There are talks of accuracy, velocity, barrel twist changes etc…But in reality, with the correct set up, with factory available match ammo coming out of an 18"- 20" barrel, none of these cited issues are ones that I have been able to prove valid. As a matter of fact quite opposite has been the case in every single rifle that I have seen that fits into this category. So next time your instructors tout the shorter barreled .308, you might want to cut them some slack. They are on the right track. 
Cutting one down to size. The right size. 

A couple suppressed Tango51’s

Tango without the suppressor. Accuracy on these rifles is truly amazing. It will shoot out to 1K with or without the suppressor without any ill effects. Provided the nut behind the trigger is screwed on right. 
