[email protected] said
WTB a 1886 Threaded magazine tube for a 30″ barrel. 

thank you much.

Mark  

It is extremely likely that you will need to find someone that can manufacture one for you. 

The threaded magazines tubes were not introduced until 1903 (circa S/N 128700), and they were only used on the Solid Frame rifles.  With the change to smokeless powder cartridges, extra length barrels all but disappeared.  In my research survey of the late production Model 1886 rifles, Take Down rifles outnumber Solid Frame rifles, and shorter than full length magazine tubes are more common than full length magazine tubes.  A post 1902 production Model 1886 Solid Frame Rifle with an extra length barrel and full length magazine tube is a very rare gun.