Hi-Force 44 works well but any line of solder that shows along a solder seam like the rib to bbl joint will remain bright silver colored even after rust bluing.
Lead/tin soft solder will darken after the trip through the rust blue process and not be noticable.
60/40 or 50/50 works fine for rib work. Most any hardware store has it. I think Brownells just wants to push their HF44 product and doesn't carry lead/tin solder.
Maybe the lead content bothers some people with the 'go-green' PC movement,,,,can you even sell it in CA?.
Gov't Warning...Don't chew on the soldering wire while working with it.
