Do cast boolits generate higher pressure?

[405]

OK. It sounds like my original thinking is correct: there is no rule whether cast boolits generate higher or lower pressures than jacketed. So just treat 'em like a jacketed and work up from the starting point.

Reading this thread did cause me to rethink another pressure sign. Flattened primers. I get them all the time - even with factory ammo. I guess if I get flattened primers with factory I didn't figure they would be an accurate indicator.

My rule of thumb is and will continue to be: Soft alloy won't shoot very accurately at higher pressures and higher velocities. So the question about higher pressures being caused by softer alloys doesn't apply to my loading. However, I suspect that softer alloys usually leads to higher pressures (let the "yabutts" begin ). I shoot for accuracy in lower pressure loads (both BP and smokeless) so I use softer alloys... and avoid the guessing game entirely.

Second thing.... flattened primers may or may not mean excessive pressure. High or excessive pressure can flatten primers OR excessive headspace at normal pressure can cause flattened primers OR lube in chamber or on case can cause flattened primers. Without pressure testing equipment guessing wrong may have consequences.

[longbow]

You know what? When I first read the question, my response would have been "Of course not, lead is softer than copper jackets." But before I spoke I thought (yeah, I know a rare moment) and decided to take a look at some references ~ mainly the Lyman Cast Bullet Handbook and Lyman Reloading Handbook.

As it turns out looking at "J" bullet loads vs. cast boolit loads, the pressures vary considerably ~ same powder same bullet weight sometimes higher with "J" bullet and sometimes with cast.

Obviously bullet/boolit shape and hardness have an effect but I did expect to see lower pressure in general with cast for same projectile weight and powder charge. Not necessarily so.

I don't hardly ever use "J" bullets so haven't been paying much attention to those load recipes but do always start with a good reference for cast when I am working up a load.

405 makes a good point ~ one shouldn't guess or assume.

Longbow

[missionary5155]

You can answer your own question by trying a little experiement, remembering that the burning powder wants to force the lead out to the side as well as down the barrel. If you have a .35 caliber barrel and a couple bullets designed to wear a gas check which are cast of clip-on WW metal, try driving one of the bullets not wearing the gas check and one wearing the gas check through the barrel with a heavy hammer and a 5/16" aluminum rod. You'll see the unchecked bullet will be driven into the sides of the barrel (in the base region) more than the checked bullet and require more force to remove from the barrel. Obturation works similar to this because of the friction between the bullet and barrel walls. Please don't forget to lube the bullets before attempting this little experiment.

MJ

Good morning There is a slight difference between battering a boolit through a bore with an undersize rod and launching it with rapid gas expansion. Your 5/16 rod does not exert even pressure on the entire base of the boolet ( Groove Diameter). You will not have complete boolet base support into the grooves of the rifling. When a boolit is fired in a cartridge the expanding gas pushes against the whole boolit base exerting the same pressure around the complete circumfrence to include those small portions that are filling the grooves. The applied Gas Check has the strength to support the boolit even out to the groves even though the 5/16 rod is not the perfect fit to groove diameter.
Friction and boolit setback do exist and will raise pressure. But also so will the lead material that begings to be displaced and wrap itself around the under groove sized rod that is battering the base.

[Bret4207]

Even the hardest lead alloy is far more ductile than jacketed. Add to that the hundreds of possible other factors...whew! It boggles the mind sometimes. I believe as a general rule cast develops less pressure because most cast is shot at lower pressures than jacketed. You can't make a side by side comparison even using the same weight boolits and and bullets and loads. They just react differently. Sometimes cast may show higher pressure, other times lower. It's the old apples and oranges thing. They're both fruit, but there it ends.

[Marlin Junky]

Good morning There is a slight difference between battering a boolit through a bore with an undersize rod and launching it with rapid gas expansion. Your 5/16 rod does not exert even pressure on the entire base of the boolet ( Groove Diameter). You will not have complete boolet base support into the grooves of the rifling. When a boolit is fired in a cartridge the expanding gas pushes against the whole boolit base exerting the same pressure around the complete circumfrence to include those small portions that are filling the grooves. The applied Gas Check has the strength to support the boolit even out to the groves even though the 5/16 rod is not the perfect fit to groove diameter.
Friction and boolit setback do exist and will raise pressure. But also so will the lead material that begings to be displaced and wrap itself around the under groove sized rod that is battering the base.

There's a whole lot of difference between battering a boolit through a bore with an undersized rod and launching it with rapid gas expansion. You're taking my little illustration too literally. I was just trying to contrast the difference in energy required to move a checked and unchecked bullet forward under the same conditions. The fact is, cast bullets will obturate at the base given enough pressure; therefore, increasing friction and in turn increasing the force required to drive them forward.

MJ

[Boerrancher]

Bottom line, if you don't understand how pressure works, and understand the signs of pressure, it is best to learn them before doing to much work at the loading bench. Also understand that each gun is its own world, and what is a low pressure load in one 357 mag, might be a high pressure load in another that is the same make and model.

I am shooting an IAB Sharps rifle in a 45-70. I have to shoot well above max loads listed for the Marlins and Winchesters in order for it to shoot well. It took me a lot of nerve wracking hours to work up to my loads that shoot well, but it was worth it. I do not recommend anyone do this. I probably shouldn't have, but there is still no signs of pressure, and it shoots well.

Best wishes from the Boer Ranch,

Joe

[Hipshot]

Felix,
How can a cast bullet obturate too much? It would have to bulge the barrel to do so. I don't understand.

Hipshot

[fourarmed]

It is a simple thing to test. Load cast and jacketed bullets of the same weight with the same load and fire them in the same gun. The one with the greatest velocity gives the highest pressure.

[44man]

It is a simple thing to test. Load cast and jacketed bullets of the same weight with the same load and fire them in the same gun. The one with the greatest velocity gives the highest pressure.

Not so, pressure has nothing to do with velocity! Have you not worked loads to find as they get hotter, velocity starts to DECREASE?
As you work loads, didn't you notice that the velocity increase was not keeping pace and was going up in smaller increments?
A perfect cast boolit can go FASTER with the same load with a jacketed bullet and actually have a lot LESS pressure.
Do NOT use a chronograph to guess pressure!

[fourarmed]

44Man, I'm sure there will be others who agree with you. We'll just have to disagree on that! Velocity comes from doing work on the bullet. Work is Pressure times change in Volume. If you think you are getting greater velocity with less pressure, you are either mismeasuring the pressure or confusing peak pressure and average pressure. There is probably some wiggle room because of differences in frictional force, but not enough to change the principle, because greater friction is also going to produce greater pressure, and vice versa.

I also believe that a chronograph is actually the BEST way for a handloader to judge pressure - by comparison with pressure-tested data from the manuals. There are just too many variables for the folklore methods to be trustworthy.

[Black Jaque Janaviac]

Work is Pressure times change in Volume.

Right, but your statement about the faster bullet yields more pressure is correct only if it takes the same amount of work to push the bullet through a bore.

I'm not convinced that copper is easier to push through a bore than lead. And I'm not entirely convinced that obturation necessarily means greater resistance compared to hardness. Having slugged a bore and played with muzzle loaders - my experience tells me softer lead pushes through easier. And copper jacketed is even harder yet.

Why do they now make all-copper bullets with grooves in them? My understanding is because solid coppers generate more resistance (and thus pressure) specifically because they don't upset as easily as lead-core bullets. So they put grooves in them to allow the displaced copper a place to go.

[Marlin Junky]

Felix,
How can a cast bullet obturate too much? It would have to bulge the barrel to do so. I don't understand.

Hipshot

As the force of obturation increases, so does pressure and friction against the barrel walls. The higher the pressure behind the bullet, the farther up the bullet shank obturation will occur and the more pressure/friction will be created between the bullet and the barrel's interior. Look at it like this: When you strike a billet of lead on its end with a hammer, it not only shortens, it also increases in diameter, greater near the end that received the blow than father away from the blow. Force vectors from the expanding gases are not just shoving the bullet down the barrel, they are also trying to expand it normal (perpendicular) to the bullet's axis... or blow it apart, even (given enough pressure). The force of obturation is always greater at the bullet base and this is why gas checks are used to help keep the base from distorting. An irregularly distorted base leaving the muzzle can only mean one thing... an imprecise shot. It's doubtful we ever experience perfectly uniform distortion.

MJ

P.S. I don't want it to sound like I believe gas checks are useful for the single function described above. There's also the scraping and all important gas-checking functions; without the latter, the lube's effectiveness is diminished.

[kooz]

I breezed through this kind of quick, so forgive me if I have misinterpreted something, but it looks like there is way to much thinking going on here. bottom line is that obturation is a GOOD thing and is the key to top accuracy and minimal leading in most handguns ( I will not add my 2 cents on cast rifle bullets as I have liitle experience with them).

[Edubya]

For every action there is an equal and opposite reaction. Pressure will find the weakest resistance, thusly creating momentum.
A hand grenade will demonstrate how the pressures work, so will a piston motor. A motor cylinder will lose power/pressure if the rings are bad and blow-by will cause destruction of the lubrication. Pistons do not obturate but the piston rings do. Hard cast bullets at lower velocities don't either. The blow-by in a bullet is much hotter than the ignition behind the bullet due to the venturi. This extreme heat will melt barrels, much less lead or copper.

[Larry Gibson]

Well, I do measure pressures. While I've not done a direct comparison so far of jacketed vs cast bullets in my tests so far the cast bullets seem to generate less pressure for any given load. That is not a definitive answer by any means as it is just an observation thus far. A direct test would have to be done to answer the question. I do have some hard cast (Lazer Cast) 240 gr .44 bullets. I might size them to equal some .240 gr Speer .44s that I have and load both over 22 gr 2400 and see what happens. Hell, with the weather I haven't got to the Blue Dot pressure test yet. You guys come up with too many interesting "testing" ideas!

Larry Gibson