Shortest barrel length with rifle length gas system?

[jurassic]

I have a twenty one inch barrel that shoots well, it has a rifle length gas system and functions just fine. I want to build another rifle, but short barrels are impossible to find. I don't like the gun because of the weight and length (it's a hunting rifle,) can I cut it down to 16" and still have it function properly? It does have an adjustable gas block.