I've used "Sack-Ups" for several years.
http://sackups.comTheir website doesn't seem to work well, so you'll need to call them. These are silicone impregnated, I believe. I've used them everywhere from Washington's rain forest to California deserts with never a problem. They won't protect from abuse or stupidity, but do keep safe dings and transportation scuffs away. They are not a replacement for good stewardship, but do add a worthwhile element of protection.
They make a couple of lengths for muzzleloaders, 66" and 72". If you're like me, you need the longer one, which I think nis #510. I ordered them in packs of 4 or 6 and saved a little. These really are a good product and worth the modest cost.
The remarks about other sleeves and cases are spot on. I've seen nice old guns destroyed by blanket sleeves, wrapping in cotton sheets, and so on. Sack-Ups are NOT the same.