Fouling is more a byproduct of barrel quality, heat and bullet lube. Mostly the first two. Trying a bunch of different ammo is really the only way to find what your barrel likes. If its nicely lapped and you don't do excessive rapid fire then you will be good for thousands of rds with little fouling. A rough, hot bore is going to lead up quick. Also, some bulk ammo is poorly lubed compared to 50 rd boxes but that isn't a rule. I find my newer 10/22 carbine barrel leads up in the throat and first couple inches of bore when it gets hot with blazer bulk but keeping it cool, I.e. keeping it away from my brother in-law, keeps leading to a minimum.