I killed my first bull elk with 6mm Rem, though the cartridge wouldn't be my preferred choice for taking elk.
Even though it's a 30 caliber, I'm guessing my grand dad killed more elk with his .30-30 than most of us put together. My grand dad worked for the Forest Service then later a lumber company so he was constantly in the hills. Family lore has it that from the great depression through the war years there wasn't a needy family in Star Valley Wyoming that suffered from the lack of fresh elk meat thanks to grandpa's .30-30.
