Hello! Hard to tell what you've got, but it looks neat. A 12pdr is 4.62 bore, so if your barrel is 4" bore, it could be undersized enough to have a 1/4" thick steel liner. Or not. The best money you could spend would be a professional X-ray to see what the breech area looks like inside. This will tell you if you have a liner or not, and what kind of thickness in the breech area, any voids in the casting, off center bore problems, etc. I had a 3" bore barrel X-rayed by a local shop some years back: it determined a very good threaded breech plug and no problems. Cost was $65. for one X-ray. I would not fire a barrel until I KNOW it is OK. If the barrel is compromised, the X-ray and possible magna-flux will save you a carriage build and possibly the death or injury to your crew and public bystanders. Some good books to get you started are "Round Shot and Rammers" "The More Complete Cannoneer" and the outstanding catalog from South Bend Replicas. Your barrel looks like a 12pdr Field Howitzer (back end) and a 6pdr 1841 (front end). It is a beautiful combination, but nothing standard that I know of. I hope your barrel turns out OK, and good luck. Artillery is a rarified atmosphere that only Gun owners and their volunteer crews can appreciate. By the way, don't forget to include women in your crew, many make fine cannoneers. I wish you well in your sojourn to a heavy metal psychosis! George. ( Ex Corporal, 1st Arkansas Light Artillery, Confederate.