Jacklex make a generic field gun for the ACW which has a choice of barrels. They might be prepared to produce some barrels for you, although my guess is that the barrels and field gun are all part of the same moulding process. Worth a call to them though perhaps?
I have previously bought Jacklex products and can recommend them. They also, incidentally, produce a nice mule-drawn wagon and a six-horse artillery team with limber, gun and riders. Being metal these are a bit pricey, but make a good 'extra' for an ACW army.
Greetings Alan, I think I have some A.C.W guns in the loft and I make silicon rubber moulds. So the next time I up there (and I have someone on the other end of the rope, that will be tied to my ankle; trust me it's not far from the truth; it's chock-a-block up there.) I shall seek out the barrels and mould some and cast them in resin for you.