A Barrel Weasel or Bore Snake works great for clearing debris from your barrel while in the field.
After clearing the snow and ice from my rifle's barrel we later caught up with some cow elk and the 243 WSSM A-Bolt did the job.
While on a cow elk hunt I stumbled and fell when my foot went into a badger hole hidden under the snow. I didn't drop my rifle, but the tip of the barrel went into the snow and packed it full of snow.
The logical first thing to do was just blow the snow out. It sounded like a good idea however with the temperature right at 15 degrees my breath melted some of the snow and the water now froze inside the barrel and chamber. I confirmed this by trying to chamber a round and it wouldn't chamber. After some time and a lot of hot air, I was able to clear the barrel and chamber of ice.
The thought occurred to me at the time "What if I got mud in my barrel?" How would I clear a barrel of debris while I am in the field? I really don't want to pack a collapsible cleaning rod with me. I found the answer with a Barrel Weasel or Bore Snake. Just drop the brass tip in from the chamber side then pull the weasel through pulling all the debris out the muzzle of the barrel and you're back hunting. These little barrel cleaners are very lightweight and can stuff into the smallest of pocket in your pack.
I don't use Barrel Weasels for my regular barrel cleaning process when I have my rifle in the man cave at home. But I do really like how small and lightweight they are to carry when I'm hunting and if I get something in the barrel I'm better prepared to clean it out.
Nov. 2010: Although I still carry a Barrel Weasel with me I also do a little preventative maintenance by placing electrical tape across the end of my rifle barrel.
Update October 2014
In 2014 I started using a Neoprene Rifle Jacket and love it. I no longer tape the end of the barrel or mess with scope covers. I use the rifle jacket and the whole rifle is protected from snow, rain and scratches. You can see more thoughts and photos of this product in Dallen's 2014 mule deer hunt.