IME, Clematis armandii is not a good choice for a container, even one as large as a whiskey barrel. Vines that grow as large as armandii and some of the other species clems also produce massive root systems - too big for a container if one expects the vine to perform as specified. I had to remove one from my old garden once and it took me days to dig it out - it was like I had planted a tree!!

You could continue to cut back periodically to maintain a smaller size but you will also need to do some repotting, root pruning and refreshing of potting soil fairly routinely, probably very 4-5 years. And don't forget to provide adequate fertilization - plants in containers need more frequent fertilizing than do those in the ground.

Very helpful information gardengal48, thank you. The armandii was my first clematis and I have loved it but now I see I meed to replace it with something smaller and perhaps a little bit showy-er.