I would guess a 6.5 mm.The 6.5 mm is about the most efficient bullet you can find and it always looks too long. I have a small collection of 6.5 mm Swede Rifles from WWI and they are very accurate even up to date. Remington also made a 6.5 mm Magnum Rifle for a bit in modern times but it did not sell well. The 7mm Magnum came out at about the same time and would work better for larger game so it won the rifle race in modern times. Remington also made the 7mm-.08 and the 6.5mm-.08 Remington, not to be confjused with the magnum calibers. There was also a wildcat caliber produced on a few custom rifles , the 6.5-.284. It was an over bore cartridge and had a tendency to burn out barrels at the chamber throat. It was never a very popular wildcat for obvious reasons but in a custom rifle it was a sure 'nuf Wild cat. Monty