I recently came into possession of a pair of these, and have never used one. I have a couple of questions...
-does this unit use the gas tube?
-if not, is there a minimum barrel length required for optimum operation?
I am thinking about building a dedicated upper with one. Any help is appreciated.
 

The Army 261 does not use the gas tube.

I don't think a minimum barrel length is necessary for optimum operation.  

I've run mine in my 16" and 20" barrels.  

Thanks.

KKF makes dedicated .22LR barrels which allow use of the 261, the Ciener and the Air Force conversions - probably will work with any conversion with a chamber insert such as the Colt and DPMS(?).

http://www.kurtskustomfirearms.citymax.com/catalog/item/29585/1131.htm

Quoted: I recently came into possession of a pair of these, and have never used one. I have a couple of questions... -does this unit use the gas tube?

No.  Compass Lake Engineering uses the M261 as the basis of their dedicated .22LR uppers.

No gas tube installed at all - helps keep the bolt cleaner.