Voids in cast bullets

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TexasMac
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Voids in cast bullets

Post by TexasMac »

In my younger days of bullet casting I always thought that voids created when casting bullets was one of the main contributors to poor accuracy (poor precision to be correct), especially voids that were formed off the central axis of the bullet. Assuming this was a significant problem I spent a good amount of time thinking about and attempting to develop a dynamic testing apparatus that one could use to screen and eliminate unstable bullets. The apparatus would spin the bullet at some ideal speed and detect the amount of unbalance. To make a long story short, I was never successful in designing a functional device. Since then I’ve come to realize that my time and efforts was wasted. The bullet casting and cooling process lead alloys go through do not create voids outside of the central axis.

I’m not implying that voids cannot exist in a cast bullet. They certainly can and do exist, but are concentrated along the central axis of the bullet. Hence weighing bullets is the best method to determine the amount or size of the voids. It would most likely be impossible to screen for bullet voids using a high speed spinning device. If you think about it, a firearm is a very high-speed bullet spinning device. Accuracy is certainly affected to some degree by variances in bullet weight but not because a void is off-axis and destabilizing the bullet. This leads to an article of mine that was published back in Oct. 2020 in The Single Shot Exchange Magazine titled Weight Variations in BPCR Cast Bullets. The article can be accessed at: http://www.texas-mac.com/Weight_Variati ... llets.html

Wayne
NRA Life (Benefactor & President's Council) Member, TSRA Life Member, NSSF Member, Author & Publisher of the Browning BPCR book
http://www.texas-mac.com
mdeland
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Re: Voids in cast bullets

Post by mdeland »

A good article Wayne. What is surprising is to shoot the culls for group and assuming they have good bases and filled out well I think one may have difficulty in telling the difference out to about 300 yards or so. I've found the weights have to be pretty close or they start showing up at the 600 yard line though.
rgchristensen
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Re: Voids in cast bullets

Post by rgchristensen »

I once did the experiment of shooting some bullets that were "light". Voids, as Wayne says, were along the center line, easily seen by grabbing a bullet just below centerline in a vise and shaving down to the middle with a sharp chisel. ANYHOW -- I shot over 50 of them (with a few full-weight controls ). Each shot was fired, scope/bench, at a different intersection on a grid, so the mean center of impact could be determined, and the displacement of each impact was plotted against the weight of the respective bullet, the weights being written on the bullets. I expected to find that, as the bullets got "lighter" they would land further from the mean center of impact. Not so!! As the weights got "lighter" (some up to 5 gr.) the excursions did become more frequent, but there were not many, maybe 10% or a bit more. As the rejects comprised at most about 5%, weighing bullets would only eliminate maybe 1/2% of the statistical fliers. Easy decision to stop weighing bullets. Weighing bullets IS useful, however, as a check on your casting technique.

CHRIS
rgchristensen
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Re: Voids in cast bullets

Post by rgchristensen »

I once did the experiment of shooting some bullets that were "light". Voids, as Wayne says, were along the center line, easily seen by grabbing a bullet just below centerline in a vise and shaving down to the middle with a sharp chisel. ANYHOW -- I shot over 50 of them (with a few full-weight controls ). Each shot was fired, scope/bench, at a different intersection on a grid, so the mean center of impact could be determined, and the displacement of each impact was plotted against the weight of the respective bullet, the weights being written on the bullets. I expected to find that, as the bullets got "lighter" they would land further from the mean center of impact. Not so!! As the weights got "lighter" (some up to 5 gr.) the excursions did become more frequent, but there were not many, maybe 10% or a bit more. As the rejects comprised at most about 5%, weighing bullets would only eliminate maybe 1/2% of the statistical fliers. Easy decision to stop weighing bullets. Weighing bullets IS useful, however, as a check on your casting technique.

CHRIS
Kurt
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Re: Voids in cast bullets

Post by Kurt »

I quartered bullets using a knife and hammer plus chucking them up using a collet in my lathe and turned them down checking for voids and I have never found one with air bubbles but I found dross trapped in the bullet. I ladle cast only so I cant say what a bottom pour pot would do that drops the alloy straight in the mould giving it a chance to swirl in the cavity causing a air bubble.

This started me to take a rag that I rubbed bullet lube on and I take that rag several times wiping the spigot on the ladle so the dross does not stick on the ladle, plus I also wipe down the outside of the mould to keep the alloy from sticking on it. I'm a sloppy caster, I dump the whole ladle full over the hole to settle the alloy in the cavity :D

I quit weighing my bullets a long time ago because I seldom found one that got close to one grain from the light to heavy end of a casting session.

Dross and a loose sprue plate will vary the bullet weights more then temperature control.
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Glen Ring
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Re: Voids in cast bullets

Post by Glen Ring »

Great Article Wayne.
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TexasMac
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Re: Voids in cast bullets

Post by TexasMac »

Kurt wrote: Wed Apr 21, 2021 7:10 am I ladle cast only so I cant say what a bottom pour pot would do that drops the alloy straight in the mould giving it a chance to swirl in the cavity causing a air bubble. Dross and a loose sprue plate will vary the bullet weights more then temperature control.
Kurt,

Good points. I only ladle (dip) cast but I know there is some percentage of BPCR shooters that continue to use a bottom pour pot. I eliminated bottom pouring from my casting process many years ago after proving to myself the significant superiority of dip casting over bottom pouring due to measuring the variances in bullet weights of the two techniques. When experimenting, compiling data and writing articles on the results I never consider the possible effects of casting with a bottom pour pot.

Wayne
NRA Life (Benefactor & President's Council) Member, TSRA Life Member, NSSF Member, Author & Publisher of the Browning BPCR book
http://www.texas-mac.com
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kenny sd
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Re: Voids in cast bullets

Post by kenny sd »

yesterday I slowed down my casting of my 408 400 BA bullets
I let the sprue cool more, and kept the temp at 700 or so.

I found that they weighed all within the 395 to 397 range. good for me.
any that I found in the 394 or lower, I sent to the pot.

and as I cast I look at the base, If it's got the 'hole' in the center, back in the pot.
but letting them cool,,,they all look very good.

sounds like I just started, but I've been casting for 40 years...sometimes you have to go back to the basics...Ken
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