max loads & velocity: whatsupwiththat?

[DxF]

I'm looking at p.152 of Lyman's 4th edition Cast Bullet Handbook: for the 311291 boolit, Mr. Lyman lists 31 gr. as the maximum load of SR4759, giving 46900 CUP (Copper Units of Pressure) and 2325 FPS; but the maximum load of SR4756 is a mere 16.5 gr. producing 35500 C.U.P. in their test barrel. The muzzle velocities are 2325 FPS and 1692.

Wait a minute! A max load of 4759 develops 46900 pressure and 2325 fps, while a max load of 4756 develops 75% of that (35500) and only 72% of 4759's velocity (1692 fps).

Ladies and gentlemen, experts all, here's my question: WHY is 16.5 gr. considered the max. load of 4756 when it develops only 75% the pressure of 4759's max. load? It seems to me that a rifle that can withstand a given pressure would be able to withstand that pressure no matter which powder was used to develop that pressure.

[mdi]

Not necessarily peak pressure, but pressure curve. I'm not familiar with either powder, but not only how high the pressure gets, but, how quickly it gets there is a consideration in load data...

[lylejb]

While I'm not sure of their exact reason, there's more to a load than just pressure.

Maybe higher loads, with that powder, were inconsistent. like excessive high / low velocity spreads, excessive high / low pressure spreads, or poor accuracy.

Not every powder's behavior is linear, maybe higher loads started to develop pressure spikes.

Without access to their test data (which we will never see) it's hard to tell why they rejected higher loads, but I do trust there was a reason.

I can't recommend going over book loads unless you have the knowledge and test equipment to develop and verify it's safety.

[Texantothecore]

The loads listed are useful loads. Upping the pressure most likely decreases the accuracy of the rifle.

Lots of loads are rejected for this reason.

[44man]

Some powders spike very quick with a small addition. The pressure curve is important and where peak is.
Don't fool with over max book loads. If you want to experiment with fast powders, know a good scrap dealer and have stock in bandage companies.

[Guesser]

4756 is much faster burning than 4759, spikes sooner and doesn't last as long. 4759 is about the same burn rate as 1680, much slower than 4756.

[Larry Gibson]

"Mr. Lyman" answers the question on page 98, section 8 of the #4 CBH. Perhaps not a detailed answer but it does answer the question.

Larry Gibson

[gray wolf]

OK This is no slam,
You don't understand it, cause you don't understand it, Sounds like a silly thing to say Eh.
I also had problems trying to understand things until I delved a little deeper into different subjects.
I had to forget the information at the top and go back to the bottom and slowly work my way up the ladder of understanding, and still it was confusing at times. But like you I got help here from those that could break things down into little segments and eventually it all came to together for me.
Great question by the way,

[Jupiter7]

This is quite common, especially when dealing with heavy end of weight range in given cartridge. When pressure curve goes non-linear, it's time to stop. But I'm in the "I don't understand" because I don't understand category also. A man has got to know his limitations and without testing equipment past a chrono, you really have no idea(nor do many of us) of what's going on in the chamber.

[hickfu]

I have asked myself that same question a thousand times but I just go with it and dont try to over charge any of my loads..... I dont know any scrap dealers and I dont own stock in bandage companies!!! :shock:

Doc