So I cast some .40 S&W using a Lee 175gr SWC die. Half of them I powder coated. Half of them I used the Lee tumble lube method. As I mentioned in another post, the PC bullets performed measurably better, producing consistent accuracy with no fliers.

Although most of the tumble lubed bullets were accurate, I had several lubed bullets that tumbled and hit the paper sideways. None of the powder coated bullets tumbled. All bullets were from the same mold and run through the same sizer. All sized to .401. All used the same powder charge. All cast from Lyman #2. I sized the lubed bullets prior to lubing them. I sized the PC bullets after baking on the PC. From what little I have read, possibly this was due to gas cutting? I do have noticeable leading in the barrel.

What is the most likely cause and should I even be concerned since I am going the powder coating route from now on?

I've had the same experience with my 38 special loads. I've been able to fix it by being careful to match the listed pressure of the load in the Lyman manual with the max pressure listed for the BHN of the bullet I'm shooting.

Lyman #2 has a BHN of 16. The max PSI listed for that BHN is 20,433 psi. So I try to find a load for that bullet that has pressure listed of around 18,000 C.U.P.. I go lower because the PSI rating is generally higher that the pressure in C.U.P. units. I've yet to find a good conversion formula between the two, but this has given me good results, and eliminated leading for me. I still do mostly PC, but have some TL bullets left over that I still use.

I'm guessing that the load you were using was a little low for the hardness of the bullet you were shooting. The lead isn't expanding and engaging the rifling. What was your powder charge?

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What could possibly go wrong?

Tumbling could be either the bullet isn't engaging the rifling..
but you know it can. It's large enough in dia to do that as the PC ones are working fine as well as most of the lubed ones.

Tumbling can also be caused by bullets (usually cast) that have voids inside them.
The bullets though they may engage the rifling OK, once out of the bbl, the lopsided weight of the void filled mass makes them wobble and striking the target can be at any angle or facing.

Weighing cast bullets is a pain, but is a bout the only way to sort them out of the mix.
The first few out of the mould are suspect for defects such as this.
When I did cast (years ago) I used to dump the first 25 to 35 bullets out of the mold into one pile and then continue with the rest in another separate pile.
Those in the first pile I either sorted by weight and figured the mould was hot and running well enough not bother weighing after that.
The badboys went back into the scrap lead pile.

Sometimes I just kept all the little beauties not bothering to weigh them. I loaded them for blasting and plinking fun shooting. Which in reality ended up being most of the time spent..

The leading you are getting may just be a combination of the lead alloy and the lube you are using on the lubed version of the bullets.
Changing lube type and/or the hardness of the alloy can greatly effect leading or lack of it.
Trying for too high of a velocity with the alloy you have will get you leading no matter the lube used though some lubes will perform better than others.
You can spend a lot of time on the range just researching that alone.

Sounds like your PC bullets are working well for you.

I never minded casting so much,,but lubing and sizing was a pain.
I did cut out the sizing and just lubed the as cast bullets. That worked out OK.
Now I just buy a box when I see them or need them!

I always thought that if the lead bullet is bigger than the bore, you should not have problems unless the velocity gets too high? I have shot everything from pure lead to hard lead and if the bullet is larger than my bore, they never key-hole. Did you measure your bore and bullets? What velocities are you shooting?

Also, there is a mathematical correlation between CUP and PSI, with a few methods out there to run the calculations, but from my research, CUP below 20,000 is almost the same number as PSI, but at pressures get into the rifle ranges, PSI numbers increase beyond CUP numbers. I keep this chart that shows the correlation of the two measurement methods.

Attached Images

CUP vs PSI.jpg (22.4 KB, 27 views)

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Gary
SWCA 2515

I would love to see slow motion high resolution video of a bullet tumbling. I hear about it all the time, but I have a hard time wrapping my head about the theory.

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Wisdom comes thru fear . . .

My load is 5.3gr of Win 231. I'm betting it was a casting or sizing issue. I did inspect every bullet thoroughly, since I'm real new to this, but it could happen.

I have no idea on the actual velocity since I don't have a chronograph. By Lyman data, I would est. It to be 950

That seems like a pretty good load for hardness of the lead you're shooting. A bit puzzling why your TL bullets are tumbling and your PC aren't when they are sized the same. Maybe the PC has some bounce back after sizing??????

I know what I would do: Keep powder coating and shooting with good accuracy.

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What could possibly go wrong?

The honest answer ... no one really knows , just another mystery of casting and reloading .
Each firearm is a law unto itself and likes certain things and dislikes others .
If you are going to powder bullets why even worry about the tumble lubed ones .
I never had much success with tumble lubed bullets except in light target loads like 38 spcl and cast wadcutter bullets @750 fps .
It might be my guns and it might be I don't hold my mouth right when loading them .
Gary

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Certified Cajun
NRA Member

Velocity or BHN have very little to do with tumbling.

I spent a year finding a solution to 9mm tumbling in a Kart barrel!

Finally ditched the .356 Lyman mold purchased a NOE .358/135 grain/RNFP mold and size to .357, lead is around 10 BHN! These solved the tumbling and also work great in 38 spl and 38 Super!

Start with a surgically clean barrel!

Try the tumble lubed bullets without sizing! I'll bet the tumbling stops and accuracy improves!

Smiles,

Just wondering if you slugged the barrel of your gun. Cast lead bullets often tumble and foul the barrel when they are too small in diameter. PCed bullets act more like jacketed bullets and shoot cleanly, not affected by gas melting. Also sometimes barrel leading will add to bullet instability and key holing. I'd say double check groove diameter of your barrel and size your lead bullets accordingly (+.002" min.)...

Forgot to mention that reclaiming a bullet from downrange or shooting into something that you can recover the bullet from will help in determining whether it made contact with the lands & grooves.

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Gary
SWCA 2515