Jerry,
I believe in the video at CW they use Borax but that would be a question for them .To give credit where credit is due Steve Bookout has taught myself and others to use 20 mule team borax but the last time I was at the Hallow Halls of Toad Hall we used Ez Weld. Personally I didn't care for it, Bookie had Angus and I use it to see how it affected the welding process. I think Ez Weld has sand in it among other things. I have welded WI without any flux but conditions have to be right. I would rather use Borax and help things along. As others have said it is a lot of work to loose all that work due to a bad weld or having to rework the barrel due to not using Flux (Borax.) Good pure WI with no junk in it is a dream to weld, but not every piece of WI is created equal using wagon rims. I am always looking for good rims , farm fields, flea markets etc. There are some good books on the Subject . Smart Dog mentioned Portuguese Book" The Espingarda Perfeyta ", I would add"The Search for the Indian Trade Gun" by Bob Heath,the book really should have been called (Fundamentals of WI and forge welding gun barrels). Both books are a gold mine of info . I have read them several times and realized later what the books were talking about when i was in the forge. Rich if this needs to split off , I realize it strayed from Old Fords original post.