Ms Chandler,

A quick look through my copy of "Texas in the War" by BG Marcus Wright, CSA, edit'd by Col Harold B. Simpson , USAF, does not indicate a Barn's Brigade; however, there are two Texas units that carried the name of Duff. One is Duff's Company of Partisan Rangers and the other is Duff's Texas Cavalry Battalion ( AKA the 14th Texas Cav Batt and later as the 33rd Texas Cav Reg. ). After looking through Stewart Sifakis' "Compendium of Confederate Armies" ( Texas) I'd say that in all liklihood, Capt. Duff's Partisan Rangers ( in the early part of the war) became Duff's ( James Duff) Texas Cav. Batt. and later on the 14th Texas Cav and then the 33rd Texas Cav. Reg.
Now remember, I'm only looking at Texas troops. Your ancestor could have joined up anywhere. If his unit was Col James Duff's outfit, there is some information "out there" on them including a wonderful description of their attrire by the famous visiting English Col. Fremantle in the book "Texas, The Dark Corner of the Confederacy" , edit'd by Gallaway.
Good hunting , Craig