I am thinking about buying a Steel Big Boy and cutting it  to 16.5 basically a trapper conversion.
I know they have the brass octagon trapper but that is not really what I want .   My concern is the mag tube and how much trouble it will be. Has anyone on here done one or know of one?
     I would think buying a replacement inner tube would be the way to go providing it is for sale ( factory trapper replacement ) But if it were not I would imagine it would be a PITA.

    The other option I am thinking is to go SBR at 13-14in (.357mag)  The Mares leg is to short besides that would be to easy

I guess this is a no

    I am starting to think a Mares leg is way easier way to get what I want

I like that idea.

I've never had a Big Boy mag tube in my hands but.....if it's the same design as a .22 rimfire cutting the mag tube at the receiver end with a tubing cutter is no big deal.  You have to cut the inner tube AT THE RECEIVER END the exact same amount, trim the spring, and press a cannelure into the inner tube to keep the follower and spring from coming out.  As long as the inner tube is brass it's easy, pezy...steel is a little harder.  They used to make a tool to do this with, but you can blunt a tubing cutter wheel and do the same thing.  The real PITA is re-cutting the dovetail in the barrel for the mag tube holder.  You really don't want to try and cut that inner tube from the muzzle end, the plug is generally both pinned and epoxied in and VERY hard to remove it without damaging it.

What's wrong with the carbine they already offer?

Rob