Can a gun smith lengthen a shotgun barrel? If you took a 18" barrel and made it 22", what would be the average cost for this type work?
If we can put a man on the moon, it can probably be done. I think it would be cheaper and safer though to simply get a new barrel.
That is the problem, I can't find a barrel for a Mossberg 3000 waterfowler. I wrote on a previous post and also called Mossberg.
If it is threaded for interchangeable choke tubes, you can get an extended choke. I don't know if any of them are as long as you want but you can add a couple of inches.
There is a process that you can do. You cut the barrel off at about 4", bore out the breech to accept stepped barrels sold by Brownells and silver solder in place.I did it with an old Fox Steringworth double.Ought to be piece of cake with a single barrel.
But if you can't do it yourself, I be ta whole new barrel would be cheaper.
You could probably buy another shotgun for the same price as having a new barrel fit to the reciever.
Like others have posted, cost would be prohibitive, especially considering it is a Mossy.
But if you are dead set to get it done, google up 'Metrogun'. At one time, barrel extensions were sold through Cabelas. The Metro barrel is a special purpose, long barrel extension that screws on the shotgun barrel via choke tube treads. If you want a 22" inch barrel, You would then cut the extension back to that length.
Probably the lowest cost option.
Howa, Smith & Wesson 3000, and Mossberg 3000 are all the same shotgun. Here's a S&W barrel, not sure if it's a 3000, but might be worth checking into:
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=226945749