You are all quite welcome for the information on Riku's barrel. I thought you may be interested in learning that there is another smith who is working on making damascus gun barrels. I've seen some photos of architectural and artistic blacksmithing work that Riku has done. He is a VERY talented blacksmith.
Could you tell us a bit about how you and the early Damascus barrelsmiths fluxed or what techniques were used to keep scale from becoming entrapped during the forge welding process?
Keith, that is an excellent question. You must have some experience with blacksmithing, to be knowledgeable enough to ask.

Honestly, it is virtually impossible to avoid slag entrapment when welding a barrel. I have a few small slag inclusions in my barrel, I think I can see a couple in Riku's barrel and Greener states in "The Gun and its Development" that it was common in the old barrels. Greener called them "grays", because these areas of slag entrapment would not color during the finishing process of the barrel.
I've read that the billets/faggots/lopins were fluxed with borax and welded in a rolling mill. The rollers would effectively squeeze the flux from between the layers as the welding progressed. I have never seen mention of flux being used during the welding of the barrels.
I've been told that the curators at the Belgian gun museums say that barrels were welded without using flux. My first reaction to this, was that this was impossible. But after further research, I have to admit that I do not have enough information to say that it isn't plausible. I am fairly confident that the "iron" element used in old damascus barrels was wrought iron. Wrought iron contains silica and is capable of being self fluxing. I have welded wrought iron to wrought iron without flux. I have never attempted to weld wrought iron to steel, so cannot say that it isn't possible to do without flux.
I've tried a lot of different fluxes and combinations of fluxing agents in my work. I've done a lot of research on welding fluxes and intend to continue. For most of my knife blade damascus, I use kerosene as a flux. But, kerosene is a one weld at a time process. So, it is unsuitable for barrel welding. On my gun barrel, I used a 50/50 mixture of 20 Mule Team borax and charcoal dust.