My friend just told me he cut his barrel on his mossy 500 to make it shorter. Wouldn't that f*ck it up?

As long as its still 18.5 inches (or 18, i forget what what you guys use, we're 18.5), or if shorter, after his tax stamp arrived, it should be okay.

It may need to be squared up with a mill, or polished up (depending on what he used to cut it), but it should work.

Barrels are cut all the time, as long as its done properly it shouldn't affect function at all.

Based on your reaction your friend is no gun smith. Did he just take a hack saw or does he know what he is doing? Sounds like it was pretty dumb.

If it is a cylinder bore with no choke threading (any it is cut square and true) it shouldn't have any effect other than making it shorter.  As mentioned, make sure not to chop it shorter than 18.5" or thereabouts, measured by droping a dowel rod down the barrel until it is against the closed bolt.

Quoted: Based on your reaction your friend is no gun smith. Did he just take a hack saw or does he know what he is doing? Sounds like it was pretty dumb.

Agreed!

He definitely is not . He used a hacksaw. What should he have used?

There's nothing wrong with using a hacksaw to cut a shotgun barrel, or some rifle barrels, as long as you know what you're doing.  Use a machinist square and a mill file to square the muzzle after the cut then use a emery cloth-rapped dowel or ceramic rod to smooth the crown.  Re-installing the bead takes a drill press unless you're a lot steadier with a hand drill than I am, getting it in the right position is the hardest part.

A rifle barrel is a little more complicated and requires the use of special cutters and reamers to get the crown right but we're talking about a shotgun barrel here, not a rifle your going to try and shoot 1/4" groups at 10,000 yards with.  As long as you can make a fairly straight cut with the saw and can use a square and a file you can do a fine job that will serve the purpose quite adequately.

Rob