Virginian-in-LA talked about making a spacer here. http://thefiringline.com/forums/show....php?p=5141359
If they aren't on the market, they will be soon. Basically, you just need a length of pipe with an inner diameter that allow it to slide over the magazine tube. Cut it so that when you tighten the magazine cap down it holds the barrel in battery. I would use a wave washer that you can buy from Brownells to ensure that it is snug but not overly tight. You can make this our of pvc tubing or pipe. I think Virginian suggested electrical conduit.
(Barrel "seated", not in battery, is what I mean.)