Better too long (humps) than too short and tears like in your post.
Some time a mold just doesn't want to perform (heat/alloy/sprue cooling cadence). I always like to (any mold maker) leave the sprue loose and let the chips fall where they may when casting. I get the mold hot and let the sprue puddle harden (low shrinkage). Then I cut the sprue's and run the sprue plate back and forth cutting the base of the bullet even (no rise).
Molds with thick/heavy sprue plates don't have this problem when up to temp.
A picture of the bullet bases of a lee 6-cavity mold. I cast/run everything so that the bullet base has a extension that gets cut off with multiple passes of the sprue plate.
[IMG][/IMG]
But this brings up when the basses are cut/flat correctly, other issues show up. Namely not enough fill out and off center holes/sprue cuts from the molds being cut and a generic sprue plate being attached to that mold. 1 out of 6 cavities hold true to the center of the mold/sprue hole alignment.