NSW Recce easy answer. Mk.12s are great from the prone and tripods, but are difficult to shoot from positions, a little ungainly to carry around and store/encase. There is no real ballistic advantage that you will see downrange between 18” and 15.1” since the projectile weights are so tiny with 5.56.
The best thing you can do with one of these builds is chamber it in Grendel if you actually shoot steel.
If it’s just for looks and social media clicks, it doesn’t matter.
From having these types of guns come through courses over the past 2 decades, I just feel like 5.56 is putting an old school 4-banger or 2.8L in a muscle car. You can dress is all up nicely on the outside, but it’s just gutless under the hood with 5.56, no way around that.
If you invest thousands into something like this, it should hit the steel hard, not take little paint chips off it and make a little noise.