I priced the job all over the country with some of the best gunsmiths around from Wild West Guns to Z-Hat (both of these premium gunsmiths agreed to do the job by the way.) I got replies of no thanks we are not interested, it can't be done, to yes we are very intereseted and would like to do the job. The highest quote I got was $600 by a well known smith (which was a cut rate for him) who said they would even get the job done in six months because they really liked the idea. One well known gunsmith told me they could not do it because Marlin uses square butress threads on their barrels and it would take special tools which they did not have to rethread the barrel after it was cut. I finally decided on two local gunsmiths to do the work and they did a magnificent job, butress threads and all. (When I picked up the gun, I commented that good gunsmiths were getting very difficult to find. He replied, "They all starved to death years ago." All together I paid $210 to have the threaded end of my 444 barrel set back 1.416", rechambered to 445 Supermag, rethreaded, reinstalled, engraved with the new caliber and properly headspaced. In addition, 2 new dovetails were cut to mount the forearm and the mag tube since the barrel was set back at the threaded end which naturally changed the placement of the original dovetails realtive to the furniture. I broke down the work and did part of it myself and split the remaining work between 2 gunsmiths. I did all the internal adjustments of the feeding mechanism myself, (which is the most time consuming and frustrating part - read $$$$ if you pay someone else to do it.) That part required a drill press, a cross vise, a dremel tool, a set of taps, and a small 1/4"X1/4" piece of steel for the extended cartridge stop. The total for all the work was still only $210 which was all labor as there were no new parts required (except the small steel 1/4" "key" I bought at the hardware store for under $2). I was able to borrow a 445 Supermag chamber reamer which probably saved me between $90 (for a rental) to $180 (for a purchase) of a chamber reamer. By splitting the work, no one gunsmith had to guarantee function and satifaction with the final project, all they had to do is guarantee their machine work. They both seemed happy with that arrangement and one even told me they would have probably passed if I had asked him to do the entire project. I did the part they hate to do and let them do the part I could not do without a lathe and a mill and the knowledge to run them.
I hope that is helpful.
Roll Tide