fknipfer,
I'm not hugely concerned with the velocity loss, which is significant, but how's the blast and the flash?
The shortest .06's I have are 20", which I find tolerable, but getting close to the edge. I have a Ruger 77 RSI in .308 @ 18.5", which I find tolerable, and I have shortened some .308's to 19" and 20". I prefer .308 when the barrel is under 20", so I wish the carbine were in a .308. But, still thinking................