0.615" would make it a 20 gauge. I'm sure that would be a challenge. I've never hunted with a flintlock, did you build the gun?
A friend of mine deer hunts with his .54 caliber flintlock, if the weather is decent. He built his rifle, it was one of these "kit deals". It's a pretty gun, I've got pictures of it someplace. Six or eight months ago, he found a side-by-side 20ga flintlock, dirt cheap. It's nearly identical to his rifle, beautiful gun.
A while back, he invited me up to his range to shoot clay pigeons, I had to work and didn't get to go. But he was using his side-by-side 20 flintlock, forget, but he said he busted 3 or 4 consecutive birds. He's a very good wingshot! I'd rather shoot at crows, they are more fun.