03-29-2020, 01:33 AM
I have seen a few ideas presented on this but there is no direct formula that I know of. First thing to consider is, it is not the max pressure per se, that drives MV, it is the total area under a (Pressure*area) vs distance (down the barrel) curve, the force acting through a distance= energy(work) performed, which leads to acceleration and then final velocity of the mass. (Max pressure figures in to yield strength, or breaking your action.) Of course, some of the working energy of the powder burn is lost to friction in the barrel and to heat, and torque (and engraving force) on the bullet from the twisting of the lands. So one barrel will have a different MV profile for a given load, than a second barrel, even if from the same mfr.
In general, one developed theory calculates that muzzle velocity is proportional to 1/4 of the peak pressure, but there are a lot of other factors in that equation too... I may try to locate that paper tomorrow and post it if I can find it again.
Second thing is that, as far as I have done, no bullet variance of COAL should be normally tried more than say 0.030-0.040" or so length, especially for non-competitive, non-ocd shooters like us. So the example of going from 2.200 to 2.290, IMO, is not a realistic "Fr'instance" -- maybe more like 2.240-2.280 might be a viable delta. In another forum one of the shooters there did a pretty exhaustive study of their bullets, powders, MV, and factored in some coal variations too. He found that the extra coal length didn't appreciably allow for a faster bullet but might in some cases lower the max pressure by 1000 or 2000 psi (he also found that it might allow a person to use a less-dense powder at a charge with that extra volume, whereas the smaller volume wouldn't allow him to load up that far, but X grs or powder usually equaled Y MV) ... this would imply that, for typical powder charge ranges, a given grainage of powder would have a certain fairly constant amount of kinetic energy to impart to the bullet for the range of COAL's you'd normally have; thus changing coal might allow the work to be accomplished with a slightly lower max pressure but the total area under the curve would be pretty much the same. This of course does not apply if you seat so far out as to jam the bullet into the lands -- that changes things dramatically!!
What the extra coal might do is to keep you from challenging the physical strength of your action, ie, the 52 ksi limits of the AR-15 bolt, lugs and lockup in the Grr, and thus limit the stress they feel during that shot.
Anyway, I hope I've been helpful, there are others on here with more experience I'm sure.
The biggest thing you can get from MV is from a ladder of the load up a range of powder weights, and observe the MV changes from one charge to the next. Bear in mind that, for a single load at each charge, there will be some deviation away from a straight linear relationship due to MV variances when you are off of a node... but a spread can give you a more or less linear approximation. What to look for is a sudden drop in delta MV or actual MV as you increase load... this can tell you you've probably reached a max for your barrel.
When doing these kinds of ladders I try to keep the delta powder charge to 1% or slightly less of case volume... so for the Grendel increase the powder in 0.2-0.3 gr increments. Then you'll be able to extrapolate a charge/MV curve for your examination. 0.1 gr delta's won't in general give you any statistically valid deviations since the MV variance can be at much smaller than the SD of that load (for several shots at that load). For a load charge that would have an SD of 10-12, a 9 ft/s increase (that you think you see) for that 0.1 gr increase is probably lost in the weeds so to speak.
Edit to add: no formulas out in the public domain, that is, there is Quickload and they have their math internally, so like Klem says, you do have that.
In general, one developed theory calculates that muzzle velocity is proportional to 1/4 of the peak pressure, but there are a lot of other factors in that equation too... I may try to locate that paper tomorrow and post it if I can find it again.
Second thing is that, as far as I have done, no bullet variance of COAL should be normally tried more than say 0.030-0.040" or so length, especially for non-competitive, non-ocd shooters like us. So the example of going from 2.200 to 2.290, IMO, is not a realistic "Fr'instance" -- maybe more like 2.240-2.280 might be a viable delta. In another forum one of the shooters there did a pretty exhaustive study of their bullets, powders, MV, and factored in some coal variations too. He found that the extra coal length didn't appreciably allow for a faster bullet but might in some cases lower the max pressure by 1000 or 2000 psi (he also found that it might allow a person to use a less-dense powder at a charge with that extra volume, whereas the smaller volume wouldn't allow him to load up that far, but X grs or powder usually equaled Y MV) ... this would imply that, for typical powder charge ranges, a given grainage of powder would have a certain fairly constant amount of kinetic energy to impart to the bullet for the range of COAL's you'd normally have; thus changing coal might allow the work to be accomplished with a slightly lower max pressure but the total area under the curve would be pretty much the same. This of course does not apply if you seat so far out as to jam the bullet into the lands -- that changes things dramatically!!
What the extra coal might do is to keep you from challenging the physical strength of your action, ie, the 52 ksi limits of the AR-15 bolt, lugs and lockup in the Grr, and thus limit the stress they feel during that shot.
Anyway, I hope I've been helpful, there are others on here with more experience I'm sure.
The biggest thing you can get from MV is from a ladder of the load up a range of powder weights, and observe the MV changes from one charge to the next. Bear in mind that, for a single load at each charge, there will be some deviation away from a straight linear relationship due to MV variances when you are off of a node... but a spread can give you a more or less linear approximation. What to look for is a sudden drop in delta MV or actual MV as you increase load... this can tell you you've probably reached a max for your barrel.
When doing these kinds of ladders I try to keep the delta powder charge to 1% or slightly less of case volume... so for the Grendel increase the powder in 0.2-0.3 gr increments. Then you'll be able to extrapolate a charge/MV curve for your examination. 0.1 gr delta's won't in general give you any statistically valid deviations since the MV variance can be at much smaller than the SD of that load (for several shots at that load). For a load charge that would have an SD of 10-12, a 9 ft/s increase (that you think you see) for that 0.1 gr increase is probably lost in the weeds so to speak.
Edit to add: no formulas out in the public domain, that is, there is Quickload and they have their math internally, so like Klem says, you do have that.
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