fouled case necks
hi, I'm reloading my .308 with 42gr of imr4064 and 168 sierra mk. My cases are all trimmed and neck sized. I am getting alot of fouling on the outside of the case necks after firing. I noticed a little of this ocuring with factory ammo but it is much worse with handloads, the fouling covers almost all of the case neck to the shoulder, does anyone have an idea of what is happening?