Poor quality picture, but you can see that the bases are egged shaped. No, it's not a fill problem. You see, even though pure lead is soft, it has high shear strength, and it maintains that strength even when it is hot. It takes a fair amount of force to shear the sprues. That force is transmitted to the base of the bullet, and sometimes results in an egg shaped base.
This particular mold would do it almost every time. Solution ? I tried casting hotter and faster, and that helped, but didn't totally eliminate the problem. A smaller sprue hole is the answer -- but it would have to be a lot smaller for this particular mold. A smaller sprue reduces the shear force, but makes it more difficult to fill out the mold.
-- there is no such thing as a "one size fits all" sprue hole. I currently stock 3 different size sprue holes to fit different applications, and it's not enough. However, it costs money to inventory different sizes. Compromises must be made. This mold was a weird application -- a 30 caliber airgun pellet. I don't sell too many of those.
-- pure lead is a pain to cast in more ways than one.