I once asked Doug Burche about leaving DBC in a bore for a long time. I had a CM barrel (oh! the horror!), and it was headed to a stockmaker's for whittling, then it would be blued. As a result it was to be more than a year before it would be finished, and I didn't want the bore to rust while it was sitting in his shop.
Doug said no problems at all leaving DBC in the bore even for more than a year, and that it would provide rust resistance when installed. He also mentioned NOT to put Dyna Gun Shield on the exterior of the barrel, because the gunsmith wouldn't be able to remove it to blue the barrel.
I put DBC in all barrels as soon as I get them, new or no. I do clean the new barrels with the same process as a used barrel, with the bore cleaner, JB bore paste, etc. Just one less thing to worry about while shooting, and I clean my rifles at a very irregular interval now.
