I hit a dead end on this one.

There was an A.C.McDonald who served in Col. Isham Chisum's 2nd Partisan Rangers.

There was also an A. McDonald who served in 1st Regiment Texas Heavy Artillery at Galveston.

I cannot find any connection between the two McDonalds, or Brig.Gen.William Steele's cavalry or his artillery at Galveston and no connection with Col. Smith P. Bankhead's Texas State Troops or artillery battery at Galveston.