you can make your mag tube longer,,i did this to a 38-55 that i had
you need a mag tube from numric gun parts or some one else,,, or buy two and save your original,,,,you will probably only use 6-8" of one of them to get the length you need
measure to see how much you need to add,,,on mine i turned the end of one and bored the end of the other for a nice press fit,,,heat up the bored end and slip the two together ,,, once they were together and straight i silver soldered them so they wouldn't come apart,,,,the forearm covered the joint
the problem you will run into will be the barrel hanger,,,on the full magazine guns,,,some have a dovetail in the end of the barrel and some use a band
if you have a dove tail now you will need to cut a dove tail in the end of your barrel to support the mag tube
if you have a band now and you move it out the the end of the barrel you may need to cut a notch in the bottom of the barrel to clear the screw that goes through the band for mag support
IIRC the end of the mag tube that goes into the receiver is turned to fit,,,and should pull out
you may contact marlin and buy a one piece tube and spring,,,,, but depending on how long your barrel is you may need to cut it to the proper length
my .02 YMMV
ocharry
