cups verse psi

[ubetcha]

I'm sure this subject has been tossed around before,but I'm trying to match my bullet alloy BHN to chamber pressures .My Lyman 358477 cast with an alloy of 2parts ww to 1 part PB has a BHN of 10.3 on my Lee tester ( as the best I can make out).According to the Lee pressure chart,I should not exceed 13288psi,but in the Lyman cast bullet book I have,the only thing that is low enough to the pressure is 8.2gr of Blue Dot at 13000cups. That is with lino alloy.If I recall,there is not a formula to convert one to the other.Anyone have a good rule of thumb as to how they would compare to each other?

[williamwaco]

Forget it.

That is a new wives tale.

The .357 magnum and the .44 magnum were developed using alloys that soft and they both exceed double the pressure number you quote.

I aim at BNH 10 when smelting but some times it is around 9.

I routinely load those bullets to significantly over 30,000 psi with no problems and have been doing so for 50 years.

You can too.

[runfiverun]

i know it.
i even have it written down somewhere.
i ignore it completley,but i know it.
if the pressure thing were true we would all be shooting linotype and the red-dot load at 40-k.
and could bump it up to fit every barell/throat/cylinder out there.

[Rocky Raab]

First, that BHN to psi formula does not give you a "do not exceed" number; it gives you a pressure at which that alloy begins to obturate. Begins to. So it's a floor, not a ceiling.

For the record, it's BHN x 1400 (that's rounded off, but the the whole idea is approximate anyway)

Second, although you can't really convert CUP to PSI, at handgun pressures it doesn't really matter - especially with this obturation formula. Use either CUP or PSI numbers as (again) the whole concept is based on a minimum pressure level, and is approximate at best anyway.

[williamwaco]

First, that BHN to psi formula does not give you a "do not exceed" number; it gives you a pressure at which that alloy begins to obturate. Begins to. So it's a floor, not a ceiling.

For the record, it's BHN x 1400 (that's rounded off, but the the whole idea is approximate anyway)

Second, although you can't really convert CUP to PSI, at handgun pressures it doesn't really matter - especially with this obturation formula. Use either CUP or PSI numbers as (again) the whole concept is based on a minimum pressure level, and is approximate at best anyway.


Rocky,

I am really glad to see you mention that. You are exactly right.

BUT.

Even though the Lee manual describes it as the pressure at which the bullet will obturate, and thus a minimum pressure only a few pages later the same manual describes it as a maximum pressure. Apparently the maximum claim is the only thing anyone remembers after reading that manual.

Another thing about the Lee information is the chart he supplies. If you reverse engineer it, is using a multiplier of 1275 to 1280 times BNH.

[pdawg_shooter]

I have completely disregarded the psi/bhn theory. If your bullet fits you are way ahead of any number arrived at by this formula. If the bullet dont fit the throats linotype will lead the barrel. Ask me how I know.

[Larry Gibson]

There is no reliable conversion of PSI to CUP or visa versa. A Mr Denton attempted a correlation and it works only in a very narrow band in the middle of useable cartridge pressures. Handgun cartridges do not fall within that band.

The PSI to BHN proffered in the Lee manual is also only a guide at best to start with. There are so many exceptions to that guide that it should not be taken as a rule. Many just ignore it. I do for the most part and have no problems without using it. If you properly work up a load (there is no magic formula that alows you to just calculate the best load) within accepted load parmeters the firearm (handgun or rifle) will tell you when the load exceeds the structural strenght of the alloy by a quick deterioration of accuracy. There are few "shortcuts" to accuracy.

Larry Gibson

[cbrick]

Yep, that formula is at best a guide, something that could be considered, a starting point. Rocky is right, it is a "minimum" not a maximum. And as was already mentioned, if the boolit "properly" fits the firearm it's not even much of a guide.

Make the boolit fit the firearm and go shooting, don't get too wrapped up with that.

Rick

[HangFireW8]

ubetcha,

CUPS converts nicely to PSI for the cartridge the CUPS measurement system was developed in... the 45/70. Everything else defies any easy conversion factor, especially bottleneck cartridges.

I was in your shoes when I started casting just a few years ago. I carefully measured all my smelting batches for BHN, blended carefully to attain exactly the right BHN for the pressure, and avoided loads that didn't have a listed pressure.

In a way it was successful, I never had a problem with leading (except with one oversized barrel). Then I discovered one of my better loads was using boolits way under the "required" minimum hardness. Some members here had already been disabusing me of the whole hardness formula notion, but then it sunk in. If my guns didn't care, I shouldn't.

Now I just load 3 basic wheel weight alloy blends, soft handgun, medium hard handgun, and hard rifle, that's all I need unless I am looking for an expanding hunting load. I still test for BHN but primarily just to make sure I haven't made some mistake.

[BAGTIC]

The 'bump up' approach was appropriate to muzzle loaders because their bullets had to be a fairly loose fit to be loaded. It has no place with breechloaders where there is no need to load too small bullets in the first place. Simple start with a bullet that is at least .001 larger than groove diameter. Even more if possible If the throat is too small fix it not the bullets.