Further to Bearhunter, the shop /gunsmith has to be 12-6.
The feds in their wisdom decided yrs ago that we did not need prohibited devices, which includes these barrels , long mags etc.
I do have a barrel, but my hands are toast, and shop closed.
I don't like to verify someone else work, though a barrel switch is straight forward, as long as the spec's are O.K., plus it has to be in my hands.
Trade Ex had K frame barrels , nos for victory models, a couple years ago. The 686 is a L frame and the threads are different.
Closely check the barrel on the 5906, While I have not check these, I have run into a couple of 39's with 105.6-106mm factory barrels on them which is restricted length.
Their is a FRT number for them, but never checked the 5906, to see if any listed.
Hi grade 12-6 stuff is still bringing good money on the online auction sale.
If you are worried about "they" taking your 12.6, I don't think so, I am too old to worry about it, that is what I use to shoot most of the time.
Posts like this, it really helps if people would include a area of the country.