There are a few spots with odd topology, which seem to be around the places where the UV map is behaving strangely.

For example, there are some interior faces:

![enter image description here][1]

Zero length faces:

![enter image description here][2]

Placing seam around the trigger seems to help a bit:

![enter image description here][3] ![enter image description here][4]

There are also a lot of ngons (faces with more than four vertices).


  [1]: https://i.sstatic.net/2QUOC.png
  [2]: https://i.sstatic.net/dF4o9.png
  [3]: https://i.sstatic.net/TpmCp.png
  [4]: https://i.sstatic.net/7HGs4.gif