Air bubbles in cast balls?

[Salty]

I have a group of cast balls that are 2-3 gns lighter than the rest. I’ve noticed that there are several flyers from this group. A fellow MLer suggested that I may have an air bubble in those balls. So I cut one in half and sure enough there is a small air bubble which I have been told throws off the trajectory. I target shoot for accuracy so this is unacceptable.
What causes the air bubble to occur? What can I do to prevent it?

 

[Phil Coffins]

It come from how you fill the mold, I found that a bottom pore pot creates the most problems and using a dipper the least. My 525 grain bullets vary by one grain using the dipper but using the bottom pore the best I can get is always much greater. How you use the dipper also is very important.

 

[RobertIN]

Keep your mold hot and pour as smoothly as you can. This looks like a case of a cool spot on the mold to me. Lead solidified on the side/top and fell into the center creating the void.

 

[waksupi]

Molds do have preferences on how the lead is poured in, also temperature. Some want shiny balls and bullets. Not me, I want them to come out frosted. That tells me I am running the lead hot enough to more compleletly fill the mold.
If shooting serious competition or working up a load, I weigh the bullets to be sure I have the most consistant projectiles. Other than that, I don't worry about it, as none of us shoot well enough to know the difference when shooting off hand.

 

[Salty]

I heat the Lee mold on a hot plate before casting. I heat the pure lead to 760. I weigh every ball I plan to shoot.
I shoot for accuracy so I try to eliminate all variables off the rifle rest. Then practice offhand knowing flyers are the shooters fault. Have not been able to eliminate that variable yet.

 

[Salty]

Do you all find steel molds or aluminum molds cast better balls?

 

[fleener]

Do you assume that you do not have air bubbles in your other balls?

 

[Salty]

Of the same diameter and similar weight (174)I do assume they do have bubbles.
I’ve cast some that are same diameter but 177-178 grains. I’m assuming they probably do not have bubbles. I’m going to test them.