I think the discussion has surfaced the issues as I had suspected, but there's also a couple other suspects that keep knawing at me.

I find that typical barrels from this era and modern barrels as well, seem to machine pretty nicely. Nothing at all like 4140 normalized (or mild steel), where you see large grain and rough machining. At least two alloying agents can facilitate free machining...sulpher and lead. Both can cause cracking in welds, if I recall correctly. Thoughts?