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:

Zero length faces:

Placing seam around the trigger seems to help a bit:

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

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

Try pressing ⎈ Ctrl⇧ Shift⎇ AltM to select non-manifold topology. Ideally ⎈ Ctrl⇧ Shift⎇ AltM won't select anything, but in your file there are some interior faces:

And some zero length faces:

Try cleaning these places up a bit.

Aside from that, placing seam around the trigger seems to help a bit: