Frosted Cast Bullet

Bullet Casting Flaws


This sounds like the dictionary but the only way I can describe a frosted bullet is to say it has a surface that looks like frost on a window pane.

The title image on this page is the worst case of a frosted bullet I have ever seen. It will be reproduced below with comments on how it was produced.

Frosting is frequently described as being caused by the mold or the metal or both being too hot. I do not believe this is true. I believe that frosting is always caused by the mold being too hot - regardless of the temperature of the alloy.  It is of course obvious that if the metal is hotter, the mold will get too hot much quicker than if the metal is cold.

The above example of a frosted bullet was created during a mold break-in session. I was having a lot of trouble getting wrinkle free well formed bullets.

I determined to eliminate temperature as a cause once and for all. I didn't think temperature was the cause but I wanted to prove it.

I started with the mold at room temperature and the metal at 650 degrees.  I began filling the six cavity mold and dumping it as fast as possible without being reckless.

I didn't time it but the cadence was about two fillings per minute.  After about 8 fillings the sprues were hardening and frosting over about like they would in a normal production run. I didn't count them but the bullets began to become show frost after about 15 fillings. At this time, the sprue hardening was more or less normal, maybe a little slow.

After ten or 15 more fillings the sprue puddle was completely liquid and would pour off the top of the mold if I removed it from the mold guide immediately. The bullets were completely frosted at this point.

After about ten more fillings, the sprue puddle was so hot it took a full 15 seconds to solidify and four or five more to harden enough I could tap it with the edge of my spoon without cutting it. It was at this point that the featured bullet was produced. 

Note that the metal temperature was 650 degrees.

This is what this bullet should look like.

It is smooth and fully formed. Note that unlike most conventional bullets the lube rings are round, not flat on top.

When casting conventional bullets, the first thing I look for when it falls out of the mold is rounded tops on the driving bands. If they are rounded, the bullet is immediately scraped off to the reject pile.
These bullets take a little "getting used to."

Unfrosted bullet

This bullet was produced at the time the frosting was just becoming severe.  This is what frosting looks like.

Note this is severe frosting of the entire bearing surface of the bullet. Most frosted bullets have only a fraction of this much frosted area.

Frosted Cast Bullet 

This is the featured bullet showing extreme  lack of fill out due to severe frosting.

This bullet weighs 233 grains. the normal weight for this bullet with this alloy is 241 +/1 one grain. The front band of this bullet measures .429/.430 It should be .430/.431.

The base band is a different matter. It measures .423.

Just for kicks. I pushed this bullet through a .430 sizing die with my pinky finger.

In some cases, very light frosting does not inhibit fill out and the slightly frosted will still be fully formed.

Frosted Cast Bullet

This is the lee .30 caliber bullet 309-170-F.

These were cast very fast at the end of a casting session for the purpose of these photos. The mold was so hot that after filling and removing the mold from under the bottom pour pot, 5 seconds later the sprue puddle was still liquid and could be poured off the sprue plate. Bullet number four, third from left was poured earlier in this casting session and is representative of normal castings with this bullet.

Bullet number one and two ( from the left) were so hot when they were dropped from the mold that they "soldered" together.  If you expand the image, you can see a flaw on bullet number two where I pulled them apart.

The average diameter of the frosted bullets is 0.0007 less than the diameter of bullet number four. This difference in diameter is not measurable in weight and if the bullets are sized, will disappear completely. The average diameter of the as-cast bullets is .3123. The average diameter of the severely frosted bullets is .3116. The largest measurement from the frosted bullets was smaller than the smallest measurement from a normal bullet.




Frosted Rifle Bullets



Frosting is caused by the mold temperature. The temperature of the metal has very little to do with it. It does cause the mold to get hotter faster so it can be a contributing factor but the ultimate cause is mold temperature.

These two bullets were cast from the same pot of metal from the same cavity of the same mold.

The purpose of the experiment is to show that bullet frosting is caused by mold temperature, not alloy temperature.

The bullet on the left was cast from metal at 500 degrees. The bullet on the right was cast from metal at 850 degrees.

As the pot heated up, bullets were cast at:

500, 550, 600, 650, 700, 750, 800, and 850 degrees. While the pot was warming up, the mold was set aside to cool with the blocks open. This was ambient air cooling. No water, no fan.  Just setting on the table with the blocks open.

The alloy was 80% wheel weights and 20% Linotype.
Temperature was measured with the NOE thermometer from

There is no difference in the level of frosting. The same level of frosting was observed at all temperatures.

frosted bullet

































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