Unfired powder will normally not rust the barrel. There have been some that conducted tests on barrel grade steel by taping different brands of powder to it, then watching for rust. The results of the test were no rust. So in the case of your barrel, I would think the chance of it rusting after being blown out with a CO2 discharger would be slim to none.

Now personally, how long would it take to put a solvent patch on a jag, run that down the bore just to make sure there is no left over powder clinging in there, crud, or anything else that does not belong. Then run a dry patch or two to dry the bore. Finally, a lighty oiled patch to make sure the rifle was fine for the evening or even a couple of days in the evet you did not get back out into the field the next couple days. Better safe then sorry in my book. A little attention now can solve a lot of problems in the future. Good luck with your unloader and keep track of the number of times if unloads for you per cartridge and how well it works...