Cavity design question

[daPhoosa]

I am considering machining my own bullet mold for the fun of it. This will be a 30 cal bullet for subsonic 300 Blackout running around 210gr.

In attempting to determining the actual size to cut the cavity (at room temperature), I have worked slightly back words.

First, it appears that most lead alloys melt at about 307C above room temp. There seems to be some variability but most alloys seem to have a linear expansion coefficient of about 27.5ppm/C (is there a better number for Lyman #2?). Since I intend to size to .310 I would like the bullet to drop at .311. Taking into account the contraction as the lead cools from melting to room temperature, I really need the mold when hot to be .3136 ( = .311 * .0000275 * 307 + .311).

Now, I don't expect the mold to run at 300C above room temp (since, in that case, it would not remove heat from the lead and the lead would never solidify). Looking around it seems 205C is reasonable running temp for a mold (actually 185C above room temp). Since my mold will be made from Aluminum, it will have a linear expansion coefficient of 25.2ppm/C. Taking the hot chamber size above, and subtracting the expected aluminum growth I come up with a room temp size of .3121 ( = .3136 - .3136 * .0000252 * 185).

Bottom line, I need to machine the room temperature size of the cavity to be .0011 larger than I want the dropped size to be. Is this consistent with other people's experience in making 308 bullet molds?

I did some searches for this in the forums, but most discussions seemed to devolve into: "... cut it and see what drops out". I'm somewhat OK with this response, but I want my first stab to be close. Why cut it twice if once will do?

Thanks,

Phil

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For those that want to do the calculations above for themselves, here is a generalized and simplified formula:

CavitySize = BulletDroppedSize * (1.0 + LeadAlloyExpansionCoefficient * LeadMeltingTempDelta) * (1.0 - MoldExpansionCoefficient * MoldRunningTempDelta)

Note: Order of operations must be observed and the units must be consistent inside of each set of brackets.

Example using this formula with my numbers above:

.3121 = .3110 * (1.0 + .0000275 * 300) * (1.0 - .0000252 * 185)
.3121 = .3110 * (1.0 + .008250) * (1.0 - .004662)
.3121 = .3110 * 1.00825 * .99534

Disclaimer: Use at your own risk. It's not my fault if you screw something up using this formula...

[Tom Myers]

Phil,

The screenshot below shows the coefficient of mold expansion and alloy contraction values that the Precision Cast Bullet Design software uses for aluminum molds and for Lyman #2 alloy.

The expansion coefficients for the different mold materials should be quite close as they are generally of a known value.
The contraction coefficients of the casting alloys are determined by algorighims that generate the closest fit to variable list of known alloy coefficients. Using the Atomic Weight ratios of the alloy elements provides a closer fit to the curve than the standard weight ratios. Much better than a w.a.g. or estimation.

You might like to consider the Precision Cast Bullet Evaluation and Design ~ Advanced software package. With the addition of the Cast Bullet Design ~ Ultimate add on, the software can do just about everything but cut the cavity in the block for you.

In addition, the software will calculated and draw a cavity design with all radial and linear dimensions of the mold cavity size needed to cast a bullet to the desired as-cast, room temperature size.

[daPhoosa]

Thanks Tom! Your software looks great. I appreciate the validation.

Cheers,

Phillip