Leave it in the horn provided the horn was well made (airtight).You don't need a bunch of humidity finding your powder, especially in the summer.
Not sure if its air tight? Empty the horn into the original jug and try to blow up the horn as you would a balloon. If you hear air rushing around and past the base plug its not a good fit. Good horn still, just don't use that one in the rain. Put the powder back in the jug. Next time seal the base plug with bees wax or hot glue as you assemble it. :wink:
I got out of the muzzleloading habit for a number of years and when I got back to it, I had BP in several horns and it worked just fine. The horns had been stored in a garage or basement most of those years. Now, I lived in Kansas and we have a much dryer climate compared to many areas, so that may have helped.
I don't have a bunch of horns...just a couple. I used to leave it in the horn, then I'd forget which powder I had in there...was that Goex 2F I used the last time or Kik? :hmm: :redface: Probably wouldn't be the end of the world, but certain guns like certain powders.
So since my brain is less apt to remember what I had in there, I have gotten in the habit of dumping it back in the can it came from after a shooting session.
I have two horns that are attached to the bag straps. They stay filled with my preferred powder. Another container stays filled with a different powder.