The current XCR Upper receiver is set up for barrels from 12" to 18.6" long. The current upper receiver sets the gas block at a certain place. One doesn't want to get the muzzle too close or too far from the gas port in the barrel. If the gas port is too close to the muzzle, the rifle won't cycle reliably regardless of the size of the gas port. If one gets the gas port too far from the muzzle, accuracy can suffer. This last statement is the reason we are not currently offering a 20" barrel. We haven't tested a 20" with the gas port at the current position but based on experiments with other rifles, accuracy may be affected. We do need to try one just to be sure.
SOCOM came up with the 18.5 for their SPR rifles which were accurized AR-15s. That barrel length seems optimum. We may at some point make a longer upper which would be more suitable for the longer barrel lengths. Along this line, we are working on a shorter upper receiver which would allow the use of 10" or 11" barrels - maybe even shorter.
When you get to the 308 barrels though, longer would be better. Like 20 to 22 inches being optimal for the longest versions.
The 16" barrels in 308 I've been around have been 'less than ideal' in my opinion. Heck, in that regard, I think the Belgians got it right with the ParaFAL - which has an 18" barrel. Much less bark and blast - surprisingly so for only 2" more barrel.
So that's what I'd request up-front with the XCR-M, barrel selection from 18" (or maybe as short as 16" for those who REALLY have to have 'em) to 22" on the long end.