This is something I have been working on for a while and the discussion of the 556SWAT and the SCAR finally pushed me to write it.
I initially did this with the FS2000 but also with the SIG 556. In any case, we will discuss the 556, this being the SIG section. The rifle itself is far more capable of accuracy than any shooter. Cooper called this intrinsic accuracy. Practical accuracy was what an operator could do with it. Practical accuracy was affected by the fit of the rifle, its trigger and of course...the sights.
In Cooper's day a scope was all but permanently mounted to a rifle and the fixed optic ruled the day as those space-age variables and red dots just didn't cut it. Today, however, optics are far more modern that anything seen in that era and the variety available is staggering. But, argue as we will, there is no one single perfect optic.
There are optics more suitable for one task or another as can be seen in the difference between the no magnification red dot, and the high magnification 6-20 variable. And of course there are compromise optics like the 1-5x.
The SIG 556 (and rifles like it such as the FNs) have the advantage of a full length picatinny rail that allows for attachment and return-to-zero removal of any optic...given a suitable mounting system...of which there are plenty.
So the concept can be "One Rifle - Many Uses", and by extension, "One Rifle - Many Optics". The optic can be chosen based on intended application, and swapped out if there is anticipation of a different need. And no...we are not talking about some sort of quick IPSC-esque optic switch, but rather a pre-planned pre-mission prep according to mission.
On the SWAT 556 I have a 4x ACOG as a general purpose scope. This is not optimal for CQB...although it can be pressed into that role (the 3X have no issues). I also have a red dot that has also been zeroed to the rifle, and is used when CQB is anticipated. Finally I have the scope seen in the photo at the GS2 class...a Leopold Mark AR in 3-9 power. Hardly the choice for CQB, but with it I can hit out to 800 yards. I supposed we could even simply go Aimpoint T-1 and Leupold 3-9 for those on a budget.