Quoted:
10" is not going to cover your gas block, You will need the 12". I know this because I was going to hvae adco install a 9" on a barrel with a carbine length gas system and they told me if I wanted the whole gas block covered I would need the 10".
9" is the shortest that will completely cover the gas block on a carbine length gas system, and 11" is the shortest that will completly cover the gas block on a mid length gas system; trust me, I have both.
Unfortunately DD does not currently offer an 11", so one's choices [among others] for a mid-length are either a DD 12.0 or an LT 11.0 if he wants to keep it as short as possible, while still completely covering the gas block.
There was a really nice pic floating around with all of the various handguard lengths [LT, IIRC] shown next to each other; I can't seem to find it, but maybe someone else will.