The 12 ga Hastings say to not shoot the copper solids due to the diameter apparently. Causes more heat and warping can occur according to a gunsmith. The 20 ga do not say anything about it. My dad, my uncle and I have owned 12 ga hastings fully rifled barrels since 96. I can't say enough about them. We consistently are shooting sub 3" groups at 100 yards with Federal Classic Sabots. All three shoot the same. My dad had owned a Remington fully rifled before and the Hastings are a big improvement over that particular barrel.