[rockpaperscissor]

Circles can be a challenge to freehand. I find I have better luck with a scalpel than I do with scissors. After modeling armor for a while, I realized that my scalpel cut interior circles (a revolving turret ring hole, for instance) came out nicer than the scissor cut exterior circles (turret tops, road wheels, etc). Just make sure that the cut line isn't obscured by the blade. I'm right handed, and I cut interior circles counter clockwise, starting on the left side of the circle. I cut exterior circles clockwise, starting on the right side. It would be the opposite for a left handed person.