Agghhh! How do I prevent "suck in" on one side?

[Jbar4Ranch]

I've got an iron 500 grain .45-70 RN design that gives me great bullets every time, but another ,45 cal iron mold, a 530 grain Postell design, very often "sucks in" on one side. I assume this is because the sprue is hardening before the bullet does and as it cools & shrinks, the material has to come from somewhere, hence it "caves in" slightly on one side. How do I prevent this?

[dk17hmr]

I would try to get the spur plate hotter or injections casting.

[grouch]

Ladle cast HOT, pour continuously over the sprue for a second or two after the mold is full. Pour at sufficient height above the mold to get good square bases. See the casting section in The Modern Scheutzen Rifle for Charlie Dell's method.
Grouch

[lwknight]

It has been my experience that when the spru plate is cold that you get a cavity right in the middle of the bottom of the boolit.
I have gad some casts get sucked in on one side and I figured out that it was the way I was pouring into the mold. I was not holding exactly level and the alloy was pouing more on one side first. That caused one side of the mold to get hotter and the hotter side was the last to cool so all the shrinkage went to that point.

If you are ladel casting then it would make more profound the pour swirl putting more heat to one side more than the other.

My other solution was to use more antimony. It counteracts shrinkage.

[303Guy]

Might I ask whether the sprue holes are the same size? Are the sprue plates different?

As lwknight says about the mold not being level. Heating the sprue funnel worked for me.