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Thread: Cast bullet bases

  1. #1
    Boolit Buddy
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    Cast bullet bases

    When I am casting 400 gr. .40 cal. bullets, no matter how I adjust the sprue cutter I get a slight bevel to the base of the bullet. It is uniform the entire circumference but it is not square or sharp. How important is a square base ?. I am thinking that when I size the bullet it might "iron out" the base. I will be only shooting out to 300 yards and most of my shooting problems are ME.

  2. #2
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    I have to ask this, but is it a bevel base mold, or a flat base mold? I'm not sure if you mean the bevel is all the way across the base of the bullet, or around the circumference?
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  3. #3
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    Check the mold and see if there is a lip all the way around the bottom by the sprue. If so, you have a bevel base mold and it's supposed to be that way. Nothing wrong with a bevel base if the bevel is even the transitions are sharp and the bottom is flat/smooth between the bevel

    What mold do you have?

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  4. #4
    Boolit Master
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    Is this a clean, sharply defined transition from the bottom to the side in one or two circles, or a rounded off edge all the way around?

    The second is a bevel based boolit, cast in the lipped mold cavity described above. The first is a flat based boolit. The last is from incomplete fill out, possibly from a cold mold or sprue plate.

    Another possibility is a loose sprue plate not cutting straight across.

    Right now I can't think of more explanations for what I've seen and done myself. Wiser and more experienced casters I'm sure will add their thoughts.

  5. #5
    Boolit Buddy
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    The molds are R.C.B.S. 40-400 CS and it appears the shear plate should cut off the base exactly square with sharp edges. I do not know what changes as sometimes that is exactly the way the bullet will come out, with square sharp edges. Other times they drop with a slightly rounded base completely uniform.

  6. #6
    Boolit Master Bazoo's Avatar
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    Several things can cause that. Not enough tin in the alloy, say if you're tryng to run straight wheel weights. Too small a sprue puddle is most likely the cause. I let the excess material in my ladle flow over the mould after the cavity is filled and it helps a lot with fillout. It keeps the sprue puddle liquid longer and keeps the sprue plate hotter. Run a large messy sprue and I'll bet you get much better consistency.

    Can you shoot them? It depends. In your application I'd guess the inconsistency would cause unacceptable accuracy, though I've not tested the difference for myself. The only way to know is to test some. Fire a group each with square, rounded, and mixed bases.

    In pistol bullets or short range rifle, it wouldn't make any difference. I use bullets with a slightly rounded base for 30-30 with gas checks and it doesn't affect me. The gas check makes for consistent shaped bases and the rounded part inside only is a slight difference in weight. Now, if half the base is square and half rounded I reject it because it would be out of balance.
    Last edited by Bazoo; 01-12-2020 at 11:38 AM.

  7. #7
    Boolit Buddy
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    By shear plate I'll assume you are referring to the sprue plate which does not cut the base of the bullet to form the base but only cuts the sprue. The beveled cutting edge of the hole in the sprue plate ought to be smaller than the base of the bullet. Consider the following causes of incomplete base fill-out:
    1. Contamination of the top of the mold or the bottom of the sprue plate. Do you use lube there? If so, do you wipe it off after spare application when cavities are full?
    In any case clean the top of the mold and bottom of sprue plate. At risk of ridicule smoke these if cleaning does not help.
    2. Sounds like the problem is intermittent. Try consistently quicker pouring/cutting cadence to increase and maintain mold and sprue plate temp. May need to increase melt temp. Pour a generous sprue puddle as already suggested.
    3. Air may be trapped under the sprue plate if too tight and not moving freely. Sometimes increased venting by minimally beveling the part line (DO SEARCH!) may help.
    4. If the rest of the bullet fill-out is good doubt that adding tin would help but 2% tin wouldn't hurt. Clean known alloy is always good (Avoid Zn.) So what is your alloy?
    5. Think outside the box: gremlins interrupting fill cadence, moon phase effect, gravity waves, etc.
    Last edited by oso; 01-12-2020 at 11:47 AM.
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  8. #8
    Boolit Master Bazoo's Avatar
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    Sprue plate lube contamination could be a factor I hadn't considered. I've moved away from liquid sprue plate lubes and use only graphite colored on with a pencil. I take my sprue plate off and color the pivot portion of the screw and the bottom of the plate as well as the mould top and alignment pins. During casting if I have a lead smear or after a long run I'll open the sprue cutter and color the bottom again with a carpenters pencil. It works great!

  9. #9
    Boolit Master

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    I use a bottom pour pot.
    I run a large sprue and after I stop the pour, I let the mold sort of drop, tap, and bounce down on a wooden block for a couple inches.

    It works to force the last bit of molten lead down into the mold and finish filling out the base on larger cal. and heavier boolits.
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  10. #10
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    You are not getting a complete fill out , probably not enough of a sprue puddle of molten lead left on the sprue hole . As the boolit hardens it pulls metal from the sprue puddle to make a complete fill out and flat base ...not enough puddle to draw from and the rounded edge base forms .

    I pressure cast with a spouted Lyman ladle pressed up against the hole and then make sure to leave a generous puddle on the sprue hole . Bottom pour pots hardly ever gave me perfect bases so I went back to ladle and pressure cast. I want perfectly flat bases and filled out boolits .
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  11. #11
    Boolit Grand Master

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    Several things might help.

    Make sure the bottom of the sprue plate is flat and in contact lightly with the blocks.Adjust it so it sits dead flat on the blocks. Any rock or lift affects the base

    Clean the blocks good and cast with the alloy on the warmer side. this helps with the fill and getting consistency. Of problem persists with a very fine stone or sand paper on a hard flat surface lightly break the top inside edge of the blocks about .005 on a side at 45*. This forms a vent line at the top of the blocks. It dosnt take alot here

    Last is to pour a big sprue pretty much cover the plate. (I ladle cast and pour a full ladle of metal letting the excess run off back into the pot) this keeps the base hot and molten longer allowing better gill out and off gassing. This is harder to do in a bottom pour pot

  12. #12
    Boolit Buddy
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    You might try pressure casting. Hold the ladle spout in close contact with the sprue plate and tip it up and let the dipper empty over the mold.

  13. #13
    Boolit Master


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    I tested some 200 SWC in .45 acp. I used the 10 worst bases I could find after the obvious rejects were culled out. My cast bullets are around 2 1/4" normally. These tested at 3" @50 yards. So yes it will make a difference but it depends on your need for accuracy. To correct the problem:
    Pre-heat your sprue plate on a hot plate
    Use a larger puddle
    Add tin to your alloy
    Fill the cavity as quickly as you can
    Up your alloy temp.

  14. #14
    Boolit Master
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    Are you using nearly pure lead? With pure lead, I cannot get perfect fill out with a bottom pour. Even with tin, on certain big bullets I do my best by ladle casting. Mold and sprue plate temp are likely your causes, but some overlooked factors are pour rate and the distance the lead is dropping. On a bottom pour, I usually end up with about 1/4" from the nozzle to the plate. For the most part you want to fill the mold as quickly as you can.

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