looking for loads in a Lyman manual, I have found that, for .308 win, some powders give velocities similar to others, but with much less pressures.For instance, with 150 gr bullets, IMR4320 gives 2840 fps/52.800 CUPs, while H414 gives 2733fps/37.500 CUPs. These, and similar data raise two questions:
1. If velocity is a function of pressure - after all, pressure is the main difference between a 30.30 and a .300 savage, is it not? - how can that be?
2. with so low pressures, 37.500 CUPS, why not pour some more powder and get more velocity?
Thanks in advance.

There are others on here that can give you a more complete answer, but let me take a stab at this, to get things started.

The "pressure" in the manuals is the peak pressure that happens during the whole process of burning the propellant and pushing the bullet out the muzzle. This happens in the first few inches that the bullet travels.

Some powders burn more quickly than others. With a slower burning powder, the peak will happen when the bullet is farther down the barrel.

The basic gas pressure laws relate pressure, volume, and temperature. If you have a larger volume, it takes more gas to fill it to a given pressure (such as 60,000 psi), and there is more potential energy stored in the compressed gas.

Remember also, the pressure behind the bullet varies with time. It builds rapidly, then "tails off" as the bullet moves. That is why the bullet picks up most of its energy in the first few inches, and relatively less in the parts of the barrel closer to the muzzle. The final muzzle velocity is the effect of this varying pressure vs. time.

The whole business boils down to coupling the potential energy out of the compressed gas without exceeding the peak PSI that would be dangerous to the barrel and receiver.

The velocity will be proportional to the integral of the pressure acting on the base of the bullet as it travels down the barrel.

A powder that burns very quickly will have a realtively high peak pressure, but cannot maintain it as the bullet moves off.

A slower burning powder will maintain the pressure mear max for a longer time, so the bullet may end up going faster for a slower powder with a lower peak pressure.

Or, "what denton said!"

Don

Graf,
I do not know if it is the case with H414 in the example you looked at, but sometimes a load is listed as the max load for a particular powder, at a lower pressure level than others, because the case is full. Manuals do not always indicate compressed loads, and I have often started working up a load only to have to switch to a faster powder when the pressure required cannot be achieved with a full case and the bullet length I am working with at the time.

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Gerard Schultz
GS Custom Bullets