04-15-2020, 01:59 PM
The key as I understand it is I think somewhat like he is saying but yo have to put all the pieces together. From a given coal (some 0.100's less than lands but still in spec) max pressure will tend to drop at some rate until you get lands contact at seating. This is from 2 things: first there is slightly more expansion volume initially due to seating farther out, causing slight drop in pmax; second there is some time for gasses to escape around the bullet before full-seating (obturation? if I get that term correctly, but full-seating in any case). However at some point when seating coal into the lands there is no gas escape during burn, so you get a large or sharp, pmax increase since there is no gas-volume loss during ignition. The final point -- and this is mostly my theory -- is that, for a given powder charge there is really only a given X amount of energy to impart, so any pmax increase (but not yet into the lands) will only follow with a decrease in the final pressures, so just about the same amount of work gets done on the bullet.
The MV, from imparted work/kinetic energy, comes from the integral of pressure vs distance, or the area under a pressure-distance curve. If the initial quantity of work available comes from the powder burn (and, assumed to be relatively constant for a powder charge) then that P*d integral will be basically constant and MV will be pretty much unchanged until you get either "very far" away from lands (more gas, ie, energy, escape) or very "into" the lands (no gas escape), these bounding conditions could show big discontinuities. footnote, I've read in another derivation that MV is more proportional to 25% of pmax, not pmax itself, thus a "2000 psi" increase might increase the proportional pressure by 500 psi... not a lot of KE difference and probably buried in other uncertainties.
I guess, as a final note, for at least some of our bullets you could be engaged in the lands before the caliber diameter fully exits the case neck... if that's true I'm not quite sure what that would mean for all of this.
The MV, from imparted work/kinetic energy, comes from the integral of pressure vs distance, or the area under a pressure-distance curve. If the initial quantity of work available comes from the powder burn (and, assumed to be relatively constant for a powder charge) then that P*d integral will be basically constant and MV will be pretty much unchanged until you get either "very far" away from lands (more gas, ie, energy, escape) or very "into" the lands (no gas escape), these bounding conditions could show big discontinuities. footnote, I've read in another derivation that MV is more proportional to 25% of pmax, not pmax itself, thus a "2000 psi" increase might increase the proportional pressure by 500 psi... not a lot of KE difference and probably buried in other uncertainties.
I guess, as a final note, for at least some of our bullets you could be engaged in the lands before the caliber diameter fully exits the case neck... if that's true I'm not quite sure what that would mean for all of this.
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