Before you consider shooting 5.56-caliber bullets, make sure you check if your Ruger Mini-14’s chamber is compatible with the caliber. Newer models have chambers that can take both .223 Remington and 5.56 NATO-caliber bullets in comparison to the older models that couldn’t. I think that the Winchester 55-Grain bullets do a pretty good job when the 5.56 NATO caliber is in question. For the 5.56 NATO ammunition, anything from 45-grain to the 62-grain range is a good bullet. For example, the PMC X-Tac M855 62-Grain Green Tip FMJ is a great armor-piercing round or the Prvi Partizan M193 55-Grain FMJ, which is much cheaper in bulk. Remember, you can shoot .223 Remington ammo from a 5.56 chamber rifle, but you can’t fire a 5.56 NATO bullet from a rifle with a .223 chamber. Except maybe if you have a .223 Wylde chamber rifle, in which case both calibers run smoothly.