Maximum chamber pressure question.

[Bloodman14]

Guy's
I just finished reading the article on LASC (http://www.lasc.us/CastBulletNotes.htm) and had a question; the article explains how to determine the minimum chamber pressure for a given alloy/BHN, but, what is indicated by the 'maximum chamber pressure? Is this the point where an alloy or lube will fail? Is it something else entirely? Some education and clarification would be helpful and appreciated.

[singleshot]

The general idea behind "maximum chamber pressure" for a given alloy is that once you exceed the yield strength of a given alloy, that is the point at which the metal will not spring back, accuracy suffers and sometimes bad leading also occurs. While this can be a general guideline for planned pressures of loads, there are so many variables that this is usually not the final answer and may not even be close. The fact that I can push paper patched dead soft boolits and 50k psi and get great accuracy seems to cast doubt on this theory. I think it's more a matter of the alloy having the strength to seal the gasses. While lube can affect how a boolit seals, I don't know of any guidelines on pressure related to lube.

Hope this helps.

[NSP64]

I think singleshot hits it on the head.
I think we can 'think too much' sometimes.
I load till I get accuracy then chrono to get drop rates.
I am not trying to get jacketed velocity from cast.
A change in alloy,
A change in powder.
A change in loob.
And its back to square one.

[Larry Gibson]

With cast bullets the yield strength of a given bullet is dependent on the quickness of the time pressure curve rather than any "maximum pressure". The faster the rise to peak pressure the quicker the alloy will hit the yield strength and fail. You want to push a cast bullet to higher velocity and maintain accuracy use a slower burning powder, all within the compatability restraints of a given cartridges capability.

Larry Gibson

[C.F.Plinker]

Does obturation occur at pressures higher or lower than yield strength?

[243winxb]

I found post 7 interesting. http://castboolits.gunloads.com/show...5425#post35425

[Larry Gibson]

Does obturation occur at pressures higher or lower than yield strength?

Lower.

Larry Gibson

[williamwaco]

A lot of people swear by this theory ( the BNH x 1422 ) formula.

Others swear at it.

I belong somewhere in the middle.

Some people say it gives the maximum allowable chamber pressure for a given alloy hardness and others say no, it is the minimum chamber pressure for obturation for an alloy of that hardness.

I have seen some evidence that it may help in reducing leading in handgun bullets.
I have noticed that I sometimes ( not always ) have leading problems with hard bullets with low pressure loads. For example a commercial "hard cast" BNH 15 bullet in a .38 special at 14,000 PSI might lead ( or it might not ) but if it does, the same bullet in a .357 case loaded to over 30,000 PSI probably will not lead in the same handgun.

However: a BNH 10 bullet according to the formula has a "maximum" pressure of 14,220 PSI. These bullets perform superbly in the .357 and .44 magnums at pressures approaching 40,000 PSI. It is clearly NOT a maximum pressure.

All these comments apply only to handguns.

I have zero experience with rifle bullets at pressures that low. All my rifle loads are significantly above the level calculated by the (BNH x1422) formula.

So, my advice is to assume it is a minimum level for any specific BNH until your experience tells you otherwise.

.

[Texantothecore]

Guy's
I just finished reading the article on LASC (http://www.lasc.us/CastBulletNotes.htm) and had a question; the article explains how to determine the minimum chamber pressure for a given alloy/BHN, but, what is indicated by the 'maximum chamber pressure? Is this the point where an alloy or lube will fail? Is it something else entirely? Some education and clarification would be helpful and appreciated.

Not sure which article you are referencing, but I will take a stab at your question.

Minimum pressure is probably the pressure at which a bullet will deform and obturate and then return to its original shape as it leaves the barrel. It is generally 10% below the readings of a hardness tester which tests for the level at which lead will permanently deform.

Start testing loads at the 10% under mark. As you get higher pressures the accuracy will suddenly fall off. Stay in the 10% range below maximum pressure.

By the way, if you are a beginning caster the obturation issue is a bit advanced for your skills and can be somewhat distracting because it is quite fascinating. When I started casting, I got caught up in this and developed fairly extensive spreadsheets with bhns of different alloys with recommended loads of my goto powders. I have no idea how accurate they are but my feeling is that they are dead on except in the higher pressure ranges.

The technique or formula below, however, eliminates most of the problems associated with obturation:

Fitting the cast lead bullet to .001 or .002 larger than rifling size nearly eliminates obturation issues and that formula is the king of all formulae. It simply works: It eliminates leading, keyholing and accuracy issues.

To start with this formula you will need to slug your barrel and measure the bore. The best video available on this subject is by one of our members here:

http://www.youtube.com/watch?v=ErFaJlUVs1Y

It takes about 30 seconds to do. Add .002 or .001 to the width of the bullet and you have the correct bullet size.

You can make small changes in the bullet width with changes in the alloy, larger changes can be made with a process called "beagling". Just look up 'beagling' with the seach function and you will find out all you need to know. Beagle, one of our members figured out how to change the width of a bullet using high temperature aluminum tape in his molds and it works well.

You sound a lot like me in your approach: Analysis is it's own reward. So therefore I will give you this advice which you should follow religiously: Stay out of the Paper Patched Bullet forums. I accidently clicked on the Black Powder Paper Patched Bullet forum and I couldn't bring myself to leave for two weeks. Fascinating stuff.

Good luck.

[geargnasher]

Guy's
I just finished reading the article on LASC (http://www.lasc.us/CastBulletNotes.htm) and had a question; the article explains how to determine the minimum chamber pressure for a given alloy/BHN, but, what is indicated by the 'maximum chamber pressure? Is this the point where an alloy or lube will fail? Is it something else entirely? Some education and clarification would be helpful and appreciated.

If you go for that, you want your maximum chamber pressure, or the peak of the pressure curve, to top out at a few percent less than the "ultimate compressive strength" of the alloy, but a few percent more than the yield strength of the alloy. Think of a rubber band: You want to operate with the band tensioned, but not to the point it breaks.

The reason, according to this method, for wanting to operate within the "flexy" point of the alloy, where it is pushed hard enough to swell a bit but not hard enough that it permanently gets deformed, is so that the boolit can obturate the bore for a good gas seal (obturate the way I use it means to create a blockage or seal, not "bump" or "slump" or "deform") and yet resist being permanently bent, scraped or damaged as it is launched through a gun that doesn't support all parts of the boolit during launch. This lets you shoot a generic boolit in a generic gun with generic loading dies with relative accuracy at low pressure and velocity. If you push the envelope past the ultimate compressive strength of the boolit, accuracy generally suffers unless you take great pains to reduce the tolerances common to production rifles intended for jacketed projectiles.

Where the pressure/time curve plays into this, especially with rifles where reasonable and practical velocities are achieved with pressures far exceeding the ultimate compressive strength of the alloy being used, is controlling the application of force until the boolit is fully engraved and supported by the barrel. If you use a fast powder that peaks pressure during the engraving event, when the boolit is usually less-than-perfectly supported, boolit damage which will affect accuracy is likely to occur. If you use a powder that burns more slowly in your cartridge, the delay in peak pressure often allows higher velocities and pressures exceeding the ultimate compressive strength of the alloy with decent results. This is why the "BHN vs. Pressure" method is not entirely useful or practical, depending on your performance needs/expectations. If you need a 28 BHN boolit to achieve the velocity necessary for practical hunting accuracy/velocity with a certain gun according to the formula, how well will such a hard, brittle boolit perform on game? If you're exclusively shooting things that are already dead, boolit composition doesn't matter to the target. You can "cheat" quite a bit by using slower powders, but eventually boolit fit will be your limiting factor.

So if you want to follow the formula, measure the ultimate compressive strength of the alloy and use load data that indicates a peak pressure of between 90-100% of that value for your component combination. Shoot for 95% to give yourself as much "fudge factor" as possible.

Gear