Phorwath,
I think there is some confusion that should be clarified first. You ask above how increasing the seating depth will result in increased pressure. I reread my post to make sure I didn't get mixed up. What I said is that making the COAL shorter or reducing seating depth will increase pressure.
I interpret increased seating depth as making the COAL longer or increasing the volume of space in which the powder will burn. Are we saying the same thing differently or do you disagree?
Regards,
Eric
Eric,
Within the realm of the seating depth changes that reloaders apply to their cartridges, my experiences over the chronographs, and research, have led me to this current understanding: that increasing COAL increases muzzle velocity, and decreasing COAL decreases muzzle velocity,
given equivalent powder charges. Setting the bullet deeper into the cartridge case, equivalent to decreasing COAL and reducing case volume, decreases muzzle velocity. Because muzzle velocity is reduced, I conclude peak pressure is also reduced.
I would agree that my experience and understanding is somewhat counter-intuitive. Unleashing the expansive gases inside a smaller case volume resulting in increased pressures would seem to make the most sense, based on the natural gas law - as applied under conditions of equilibrium. But I believe that what is experienced in real life, is that any increased pressure resultant from the slightly reduced case volume due to seating the bullet deeper into the casing and decreasing the COAL (which would decrease case capacity and be expected to increase cartridge pressure) is more than offset by the reduction in force (and consequent required pressure) by giving the bullet a running start before it engages the rifling - in the bigger picture. In other words, 'increasing' COAL (increasing interior case volume) may actually decrease pressure
prior to the bullet engaging the rifling. But the peak pressure experienced as the bullet engages the rifling is increased an even greater quantity, than any reduction in pressure experienced prior to bullet to rifling contact. Such that the net result is increased pressure and increased muzzle velocity.
Consider this to be an extension of the consequence of jamming a bullet into the lands of the rifling, which many of us have experience with. Increasing COAL until a bullet is jammed into contact with the rifling by 0.010" will notably increase cartridge operating pressures, compared to pulling the bullet away from the lands 0.010". With the bullet jammed into the lands, the additional friction and resistance to acceleration caused by the bullet being swaged into the lands causes increased peak cartridge pressure. Decreasing COAL (seating the bullet deeper in the cartridge case and separating it from contact with the lands of the rifling) allows the bullet to start accelerating free of the lands, and by engaging the rifling with a running start, the force required to start the bullet into the rifling is reduced. This is analagous to a lesson from one of my college classes: the force required to keep a moving object (bullet) in motion against another object (the rifling/bore) is less than the force required to initiate the motion of the same stationary object in contact with the other (bullet jammed into contact with the rifling). Stated another way, the coefficient of friction between the two stationary objects is greater than the coefficient of friction between the objects after one moving against the other.
Another way to conceptualize this is by considering how Weatherby freebore chambering allows for increased maximum powder charges and increased muzzle velocity, compared to the same rifle barrel and cartridge chambering without that increased freebore. Giving the bullet that running start prior to engaging the rifling reduces the peak cartridge pressures so much that additional powder is required in order to re-establish maximum cartridge pressures, and experience the resultant higher muzzle velocities. The increased powder charges and muzzle velocities aren't a byproduct of the increased cartridge case volume (or additional volume gained by the freebore - removal of the rifling lands). They are a consequence of the reduced friction and reduced back-pressure realized at the moment the bullet engages the rifling, because it enters the rifling at higher speed and with less friction.
That's my take on it. Bryan or someone else may be able to express this more eloquently than I can, and add, or remove, validation from my explanation. I know this issue is commonly misunderstood, because differing positions are so commonly expressed on the Forums. I began to research and analyze this subject after my chronographs repeatedly recorded slower muzzle velocities when I decreased COAL (reduced case volume by seating the bullets deeper in the cartridge case - farther away from the lands of the rifling).
Appreciate your sharing the facts on this Forum, as you understand them, with respect to the design, production, and troubleshooting of the .338 Hybrids. I hope my Posts don't detract from the purpose of this Thread, or discourage your continued participation in it.
Good shooting!
