The Collaborative International Dictionary of English v.0.48:

But \But\, n. [See 1st But.]
   1. A limit; a boundary.
      [1913 Webster]

   2. The end; esp. the larger or thicker end, or the blunt, in
      distinction from the sharp, end. Now disused in this
      sense, being replaced by butt[2]. See 1st Butt.
      [1913 Webster +PJC]

   But end, the larger or thicker end; as, the but end of a
      log; the but end of a musket. See Butt, n.
      [1913 Webster]