What must be done, and how much to do it?

I'd bet that the original 45-70 ELW barrel is too light to rebore to 50.

The alteration may possibly require opening up the loading port on the inside and WILL DEFINITELY REQUIRE somehow ensuring the mag tube and forearm remain in place under recoil. I assume that the modern 1886 has a screw-in mag tube like the 71s, which has proven adequate to restrain forward movement. However another bugbear is separation of the forward end of the mag tube from the bottom of the barrel. The inertia weight of the cartridges in the magazine combined with the overwhelming whip of the barrel under recoil is guaranteed to cause the pitiful little dovetail tenon to pull apart. Guaranteed.

How do I know? 2 original 71s rebored to 450 Alaskan plus many original ELW 86s in 45-70. Knock your wee-wee in the dirt with a crescent buttplate!

Nowadays I convert to a modified SRC style, with full-circle band(s) and cross-screws. My 'fight-stopper' original 86 (with Shilen bbl) pushes a 535-gr @ 1725 with less than a full load. It weighs 8 lbs and I don't shoot it much at all...
Regards, Joe

JD:

Thanks. Sounds expensive.

Bill
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Heck if you are going to rebarrel an 1886--- might as well go to the 50-110 instead of the 50 alaskan. Modern smokeless loads in an 86 can be an extra 200 fps more painful!
Not that the 50 alaskan is a creampuff to start with, especially with a steel curved buttplate!