Velocity and RPM limitations with cast bullets shot from modern rifle cartridges

Casting your own bullets is hobby filled with all sorts of self taught experts willing to offer advice over the internet. A thank you to Larry Gibson for being the most vocal proponent of the “RPM limit” for cast bullets, which inspired me to write this post.

Back when the “velocity craze” was big, and the 22 eargesplitten loudenboomer was blowing up jacketed bullets in a puff of metal between the barrel and the target, people started realizing that you had to ensure your bullet was tough enough to make it to the target. A few years before that, Roy Weatherby made magnums that shot bullets so fast they just “exploded” on impact, causing hideous surface wounds but failing to reach the vitals on game animals. The Nosler Partition bullet was introduced about 4 years after the 300 Weatherby Magnum, giving a commercial bullet tough enough to stand up to the forces generated by the big magnum.

Cast bullets are less structurally sound than their jacketed counterparts, so for the same barrel twist and bullet weight you can drive a jacketed bullet faster because it can withstand the centrifugal forces because the jacket holds the lead together (or some bullet designs use mild steel, or eliminate the lead altogether).

So when you are shooting cast bullets in a modern rifle with a rifling twist rate made for jacketed projectiles at high speeds, you’ll hit a point where the lead deforms because it is spinning too fast and your accuracy goes straight to hell. Empirical testing puts this “RPM limit” between 120,000 and 140,000 rotations per minute for normal lead alloys according to Mr. Gibson. The reason for this 20,000 rpm range is that cast bullets are lead alloys, and different alloys will fail at different RPMs.

There are things you can do to the alloy to make allow it to shoot faster. Adding copper to the mix, or Babbit metal seem to be two of the more popular choices.

The other option is to figure out how to get the performance you want within the velocity limitation you have for your rifle and lead alloy. Or if you are still shopping for a rifle you can find one with the slowest acceptable twist rate to increase the velocity while staying below the RPM failure threshold.

To use the M96 Swede Masuer with a barrel twist of 1:7.87, K98 Mauser with a barrel twist of 1:9.45, and 30-30 Winchester with a 1:12 barrel twist and a 222 Rem with a 1:14 twist as examples, assuming 140,000 RPM limit for the bullets, the Swede will have a velocity cap of 1530 fps, the K98 at 1837 fps, the 30-30 at 2333 fps, and 222 Rem at 2722 fps . Using the right cast bullets and assuming the same ballistic co-efficient, this makes the 30-30 a much better candidate for cast bullets than either the Swede or K98 Mausers.

Now, this isn’t to say that you can’t go faster, just that it’s not easy. The other thing to do is accept a lower velocity and have fun. If you want to hunt with that old K98, a 225 grain flat nosed cast bullet will hit like an anvil at 1800 fps and depending on the alloy you use, penetrate deep.

Now a slower twist barrel seems like a great thing, but the slower the twist, the shorter your bullets must be to actually be stabilized in flight. So cast bullets are either a “heavy and slow” solution or a “lighter and faster” solution based on your barrel twist and powder capacity. If you want to shoot lighter bullets at moderate velocities for varmint or small game, you are in great luck. As far as hunting goes, there are more squirrels and rabbits for hunting than deer or elk, so no point wasting a perfectly good big game bullet on small game. For that old 222 Rem with a 1:14 twist, cast bullets makes perfect sense for a squirrel/rabbit load. For the 30-30, a cast 170gr flat nose bullet at 2200 fps has proven itself as a venison getter for over a century.

But, you won’t know what the velocity threshold for your bullets is until you load ’em up and shoot ’em. Until you do that it’s all academic.

I purchased a Lee 8mm Maximum bullet mold just because it will give me a real heavy cast bullet that is suitable for big game hunting at reasonable ranges from a K98. I have yet to try it but really look forward to all the load development, my accuracy goal is 1.5 inches or less at 100 yards (essentially a WWII sniper rifle level of accuracy). It won’t be a long range load by any means, but should keep up with the sedate 30-30 just fine in the woods.

Remember handloading doesn’t save you money, you just shoot more. Casting bullets is just the same.

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