Recently I've seen hunting shows and movies with Glocks that have barrels extending out from the housing about 2".
Why use a longer barrel?
Does it effect accuracy? . . . Velocity?
Who makes them?

I know if you are hunting in NC with a pistol the barrel has to be 6" long. Most semi auto's are not longer than 5" long, so the extended barrelwould allow them to be legal here.
You should be able to get an extended barrel here
http://www.glockworld.com/barrels.htm

Some of the longer barrels are ported.

You may get 50fps more velocity.

threaded barrels are also longer than the frame and standard barrel.....

movies it could be something for the blanks maybe?? or porting for more flash?

aFTER MARKET EXTENDED BARRELS will give you more vol.and better,longer site rad.Another benifet is you can shoot lead.pan.
PS.Excuse caplocks.

The extended barrel on a semi will only give you an extended sight radius IF you have an extended slide to go with it.

How does a longer barrel make it easier to shoot lead bullets?

The thing is most of the longer replacement aftermarket barrels offered for Glocks offer these advantages IF you have to have an extended barrel: Usually ported (reduced recoil)or threaded (suppressor or muzzlebreak), supported barrel instead of factory unsupported barrel, usually conventional rifling instead of factory polygonal rifling enabling use of cast lead bullets.