Sorry, I'm a little slow...any of you sending emails will also know this!
Those barrels are only available with a carbine gas system in a 16" heavy barrel right now. I say available, but we really don't have any in stock right now. We've got some on order but we've told our supplier to concentrate on the standard 5.56 barrels, so I'm not sure when they'll be in!
Let us know if you have any other questions.