Design myths

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Design myths

Post by mtngun » Sun Feb 17, 2019 7:15 am

There are many myths about selecting or designing cast bullets. In the internet age the newbie typically goes on an internet forum and asks the experts for advice. Except the "experts" are not really experts, and they give bad advice. That includes, but is not limited to, the CBA.

"First thing, you should slug your barrel." Horrible advice. All kinds of things can go wrong when slugging a barrel, and even if a satisfactory slug is procured, the average person does not have the tools (a micrometer) or the skill to measure it reliably. Further, most of the time there is simply no need. Was your barrel made after 1960 by a reputable American manufacturer? If so, it's probably within 0.001" of spec, so why bother measuring it? There are some exceptions to the rule which we'll get to later.

Due to abrasion, a soft lead slug typically comes out smaller than the actual barrel dimension, and if you repeat the procedure, you may notice variation from one slug to the next.

But it depends on the alloy and BHN. A harder alloy will not abrade as much as a soft alloy, and may even "spring back" after it exits the barrel.

While most name brand American barrels are reasonably consistent, some of the cheaper brands, and some of the imports, are not. Marlins are known to have tight spots under the letter stamping and under the dovetail cuts. Some revolvers have a restriction where the barrel screws into the frame. But if the barrel has tight spots then a slug pushed all the way through the barrel will be misleadingly small. An experienced cast bullet shooter might be aware of those issues and "feel" for tight spots, but I would not expect a newbie to do that, and even an experienced person can have a difficult time "feelilng" tight spots unless he has a sturdy vise to hold the barrel while slugging.

I've heard of Savages with varying dimensions, and NEF's with oversize dimensions. In the case of those low quality barrels, then yes, it may be worthwhile to slug the barrel, but otherwise I rarely bother because most name brand American barrels are within 0.001" of spec.

Another exception to the rule is black powder cartridges. In the old days tolerances could be sloppy, and even modern guns chambered for black powder cartridges like 45 LC or 38/55 can have odd dimensions.

But even if by some miracle the newbie procures an accurate measurement of the barrel, the optimal cast bullet dimensions will probably be determined by the throat and the chamber, not the barrel, so why bother slugging the barrel? :roll: If you are determined to measure something, then measure the chamber and throat.

"Design a bullet that will be a glove fit in the chamber." Mostly bad advice. It is true that CBA benchrest shooters go to great lengths to produce a cast bullet that is a glove fit in the chamber, but they put a lot of effort into it, and even then, their tight fitting cartridges don't always chamber or extract reliably. The average caster does not want to nose-size his bullets for a glove fit, and the average shooter does not want a cartridge that fits so tightly that it is difficult to chamber or that sticks the bullet when attempting to extract an unfired round. That type of "glove fit" has no place in a load used for hunting, plinking, or self-defense.

9 times out of 10, the best design for the average shooter is a single diameter bullet that drops out of the mold 0.002" oversize and then is final sized to fit the chamber snugly yet not so snug that it is ever difficult to chamber or extract.

Sometimes a single diameter bullet won't work in a particular gun and then it is necessary to use a 2 diameter bullet. Just be aware that unless you are willing to nose-size that you will be at the mercy of the as-cast dimensions of the nose and all the possible problems that entails, so never use a 2-diameter bullet if a 1-diameter bullet will do.

There is an exception to that rule and that is when the nose diameter is not critical. One example is 2-diameter revolver bullets -- the nose diameter is not super critical as long as it is bore diameter or slightly over. Another example is for 45/70 lever action and other relatively short nose lever action bullets -- the nose diameter can be slightly under bore diameter and that's OK because the nose is not long enough to make its fit critical. For a lever action hunting bullet, err on the side of reliable functioning.

On rare occasions it may be desirable to have a 3 diameter bullet, but the KISS rule still applies -- don't use a 3 diameter bullet if a 2-diameter bullet will do the job, and don't use a 2-diameter bullet if a 1-diameter bullet will do the job. If you do opt for a 3-diameter bullet, ask yourself how you are going to size all 3 diameters? :? You say you expect the diameters to be perfect when it drops out of the mold? Har har har. :lol: :lol: :lol: Even custom mold makers have tolerances, and even the best molds will vary in operation due to temperature, alloy, casting technique, or lead splatter on the mold faces. The only sure way to guarantee bullet diameter is to size the bullet, and sizing has it's own challenges with variations.

This is by no means a complete discussion of design myths, it's just a couple of pet peeves. Keep it simple. Most of the time you size to fit the throat or chamber and pay little attention to the barrel. Most of the time you want a bullet that drops out 0.002" oversize and then you can size it down, and the 0.002" oversize gives you room to experiment with different diameters because at the end of the day that's the only sure way to determine which diameter is best for you situation.