Lyman 45240 voids in the base continued

[bpost1958]

I have been using this mold for 30 years and have seen the void hundreds of times, at 25 yards it does not matter. At fifty yards it is a group wrecker. The mold is a tad tired but still tight, venting in the blocks is fine, 95% of the boolits are fine, it is the 5% that has me worried.

For some reason this boolit style can create a void in the base on some small percentage of casts. Some look like little pin pricks near the sprue, some are a tell-tale indentation on the outer edge of the cast, near the edge of the boolit. All of them are larger cavities than meet the eye.

There is little evidence of the deeper voids until the boolit is fired. These voids MUST (and do) upset the flight. I discovered four tonight that LOOKED absolutely perfect but the air pressurized under the boolit during sizing indented the small void present. Lachmiller sizer 460 top punch Alox/beeswax .452 sizer.

After painstaking care today, in 90 degrees plus heat and high humidity, I cast 20 pounds of them for an upcoming 2700 Match in VA.

The issue is NOT pull out or mold venting. The pour rate was varied from a heavy flow to a trickle from an RCBS pro-melt. The void, once you know what to look for can be confirmed by a dental pick. The void is USUALLY oval in shape varying from almost the size of a period(.) to as large as 1/64th X 1/8th in length. The formation is RANDOM with no discernible pattern. Fifty Boolts may be fine and the void develop in six out of the next ten boolits cast.

Please note, the alloy does NOT matter in the formation of the void. The MOLD used does not matter, it is the Boolit, the base specifically that has the "issue". Other shooters, picking up range lead have noted this dimple too. It is not an isolated case. The Lyman 452460 Mold shape is the common denominator. I also have a Lee 200 SWC bevel base six cavity mold. That mold does not exhibit this trait.

What do you think is causing this void????

Another question; why are boolits poured base up? One would think the base is the most critical part and therefore be on the bottom where perfection of shape is almost guaranteed.

[cajun shooter]

Your last question is answered in the fact that most all custom mold makers build nose pour rifle molds for bullets that are intended for the likes of Creedmoor. If I was you I would add some tin to your alloy and see what happens. If that does not help. I would then buy an electronic scale so that I could weigh all the bullets poured and do away with the ones that weigh light. That will at least put your mind at ease for the up coming match. If you go to shoot and you are thinking about a bullet void ; how well do you think you will shoot?

[looseprojectile]

any number of things.
Is the sprue plate loose enough?
Are the holes in the sprue plate large enough?
Are the mould halves chamfered a little on the inside top edges?
More heat.
More flux.
Some tin added.
Sounds like a venting problem along with a flow problem.
By blaming the shape of the boolit you may be overlooking something that will fix it. I would guess you have a two cavity mould. Boolit shape probably has little to do with fill out.
I run my iron moulds hot enough to be able to bump the mould on the bench before the sprue sets up to settle the liquid metal in the cavities.
I think some more heat on the mould and the melt will help.
I have experienced your problem with long large rifle boolits and solved it with one or more of the suggestions I have given.

Life is good

[AZ-Stew]

Try leaving a generous sprue puddle when you cast. Allow it to cool. Watch the part of the sprue puddle that's over the sprue plate hole as the melt cools. You'll see it sink down, leaving a dimple over the hole. This shows that the metal from the sprue puddle is being sucked down into the mould as the boolit cools and shrinks.

If there's no puddle to draw from, the boolit will suck air through the sprue plate hole as it cools, resulting in the base void you are seeing.

Regards,

Stew