Bad venting, too loose sprue plate, low temp, wrong alloy, etc. No matter the cause, I don't use such bullets (what's a "boolit"?) even though the gas check covers it. A partially rounded edge equals an unbalanced condition no matter if it can be seen or not, and as such will not fly true- to one degree or another. Remember, the base of the bullet is the steering end, and any anomalies there are bad juju. If the bullets have a perfectly uniform rounded bottom edge, that would be a different story- but that ain't gonna happen.
