The only time I have had a misfire with 777 wasn't due to the moisture getting in the powder. I was hunting in the rain all day a couple of years ago and when the rain ended, of course a deer moved through the area. I went to take a shot and the 209 primer didn't go off. Moisture had gotten into it. By the time I recapped and recomposed, the deer was gone. It was a doe, but I was still pissed. I now basically do what manufacturers do to the primers on their waterfowl shells. I paint them with lacquer. You can use clear finger nail polish or spray them with spray paint clear coat. Keeps them sealed. I haven’t had a problem since. I also cover the barrel with plastic wrap and a rubber band when hunting in the rain or if it is really foggy or dewy. You can use finger cots, condoms or even balloons. The heat of the powder burns through it. Doesn't affect the shot. Keeps your powder dry!!

If you weatherproof your barrel, keep moisture out of your primers and store your powder properly, you shouldn't have a problem.