I just put a piece of electrician's tape across the muzzle when hunting. It not only protects the crown somewhat, but keeps dirt, snow, and rain out of the barrel. Air in the barrel ahead of the bullet blows the tape away before the bullet reaches it, so there's no variation in bullet impact.
The crown isn't the only part of the bore that's critical. The last inch of bore is the second most important part. Rifles that are cleaned from the muzzle end, (especially those with micro-groove rifling), can be worn by the grit that accumulates on the cleaning rod. I liken it to a Remington Rod Saw, but to a much lesser degree. Perfectly straight, polished stainless steel rods, wiped after every patch or brush sroke will cause less wear than softer rods. It's not the rod, but what sticks to it that will cause the problems. Primer grit, especially when mixed with metal fouling, can stick to coated or soft metal rods.
Bolt-actions are cleaned from the breech end and rods should just clear the muzzle, to minimize wear at that critical location. Brushes should be removed at the muzzle and never dragged back across the crown. For that reason, I grind off most of the threads on my brushes, so a quick twist removes them after they clear the muzzle.
Hunters who shoot once a year and shoot a deer, then clean again will probably never wear out a bore. Hunters bores usually rust/pit from lack of cleaning and protection. Hoppe's #9 bore cleaner is not adequate protection. I use Break-Free after cleaning with other products, but it also cleans quite well.