I have been using a "C-shaped tunnel" in weave training for my young dog Tux, decided to replace it with a barrel in my second training session this morning just to see what would happen. He has seen (and likes very well) quite a few tunnels -- this wasn't the first time he has seen (or been sent around) a barrel, but the first time in this context -- and he got it on the first try!
Here is a link to some "real time" video from training this morning (less than a minute and a half). I didn't edit out the "missed barrel" on the second try which shows what can happen with a young or inexperienced dog if you assume that he is "committed" to the barrel and quit drawing the path for him! :^) We got it right again on the third try. I'll probably trade off using the tunnel and barrel for this training purpose, but it doesn't appear to me -- at first glance -- that he is any less enthusiastic or slower using the barrel to make this turn than the tunnel. Hope this link works!
Happy to answer any questions anyone might have.