Friday - June 25, 2010
.45-60 Musing: Throat Length, Pressure Curves, and Powder Selection
This is the same issue I was writing about last week, but looked at from a different perspective. Last week I was wondering how the throat length would change the peak pressure curve, and by how much, and musing to my self on what to do about it. I sent the drawings I had made off to the technical guys at one of the gun powder companies, and his reply was that the longer length cartridge would have lower pressure and velocity because of more internal volume. That I understood.
The other question that he didn’t answer was whether “longer length” was limited to actual physical length, or “effective” length ... and that’s what I really wanted to know when I asked that question about throat lengths. But I think the real reason he couldn’t answer my question was because I asked it wrong. This is what I asked:
Right now, I’m thinking that they might mean the same. But I should have asked the question this way:
Experience shows me that this is true, but I was looking to see if the shift could be calculated based on a powder’s burn rate or it’s amount of contained energy. In other words, I wanted to know the how much part as well. Because if the how much is quite a bit, then that means that the load that I thought might be optimal will be reduced considerably (see above graphic), and the load that I thought was too hot then becomes more optimal:
I have hidden the actual powder charges in these calculations, because I don’t fully understand what’s going on, and I don’t want to put unsafe data out on the internet. The calculations my bit of software came up with may be off by a mile, and the difference in OAL may be seriously overstated ... so it would not be responsible to say “X grains of IMR4198” because that could be a lethal mix. All I can say is that the general theory is correct, my idea about throats may be correct and part of that general theory, and if my ideas are right, then my approach - use a faster powder, as shown in picture #2 - may be the right one. Only an actual pressure testing rig can find the truth, firing the same loads in the same barrel both before and after a throat is cut for the chamber. But I may be able to deduce some of the truth by building loads that match the calculated “short” maximum pressure, and seeing which one is closer to reality when tested. I also know a gunsmith who is an expert on throats, so I can ask him. Hmmm.
Posted by Drew458
on 06/25/2010 at 11:13 AM
Filed Under: • Guns and Gun Control •
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