Zapata

A simple way to build your own double is to start with a double rifle.

Buy a Pedersoli Kodiak in .45/70.

Have the chambers rebored to your .45/120 or .450 No2 NE.

And you have a nice cheap double rifle with a better cartrdige than the .45/70.

If you are in the USA there is a gunsmith that has done this successfully several times and the rifle has not required re-regulation. Of course it may just need it after rechambering .

The Pedersoli does have exposed hammers but that wouldn't bother me. Very 19th century.

And this is the beast.