Here's a useful tip for keeping your gun dry in a driving rain.
Grab some of those free umbrella bags many stores have on a rack at their door.
I keep several in my pack - rolled up and secured with rubber bands.
Two will cover a gun perfectly. Cut them to fit - one put on at the muzzle end and a second one cut off to overlap the first one by a few inches. After putting them on the gun, I secure the rear one with a rubber band around the wrist of the stock. It takes only a second to slip the front one from the barrel.