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Explanation desired
hadjii
Member Posts: 976 ✭✭
O.K., so I'm looking at my Nosler reloading manual and doing some research on my 264 win mag, and read that the 7mm Mag is so much better.I can accept that the 7mm is better because it has a larger variety of bullet weights, but here's what I don't get. The 264 win mag and the 7mm mag is the same case. Alot of my brass is 7mm Mag brass necked down to .264. For a common bullet weight(140 grain), in a 7mm Mag, the max load using IMR 4831 is 66.0 grains, and in the 264 Win Mag, the max loading is 59.0 grains. I can't believe 264 dia bullet would have to be seated that much deeper to take up 7 grains of space. In the Lyman manual, they load the 7mm Mag to 59,000 cup and they load the 264 win mag to 51,000 cup, using the same powder, the same grain bullet, and the same universal receiver. Obviously, loading to a higher pressure will yield higher velocities, but I just don't see why you can load the 7mm to a higher pressure than the 264, when everything else is equal. Can someone explain this? Thank You
Comments
With all the bottom feeding liability lawyers around nowadays compared to the 60's, when the .264 & 7 mag were developed. The folks publishing the current reloading manuals, are going to err on the side of caution.
IMHO the 220 Swift and 264 are hard on good barrel life when loaded on the top end.
I only use ball powder in my 162 BT loading for my 7mm Remington Mag, cooler flame temperature so they say.
One of the considerations in load data is pressure spikes. Some cartridges are worse than others, when it comes to the tendency to throw spikes.
The pressure you see listed is the Max Average Pressure (MAP), there is another spec, Max Individual Pressure (MIP)
When they do the testing for the manual, they run 20 shot strings, and look at the variations. If any single shot is pushing near the MIP limit, they lower the MAP for that powder/bullet/case to insure safety.
A different powder, IE one that's more consistant in pressure, can run a higher MAP (assuming that SAAMI limits are still not being exceeded, of course).
BTW, higher pressure does NOT mean higher velocity. Velocity is a function of pressure over time. A lower pressure peak, with a higher average pressure can (and usually does) yield higher velocities
OK. So a 264 dia bullet of 140 grains will have a longer bearing surface than a 140 grain bullet of 284 dia. So that would explain why(I think)that a 264 would see sharper presssure rises than a 284 dia. That still doesn't explain why the 7mm Mag is loaded to a higher pressure level than the 264 win mag.
I have been a 264 shooter for 35+ years, barrel life is a big problem, don't make it any worse.
W.D.
I've finally gotten to where I just roll my eyes at what (gun)writers say about a cartridge they are trying to introduce or talk about. I've heard so many comparisons that when it comes right down to it, the (gun)writer couldn't seem to make that popping noise and figure out how to make the necessary changes. The most glaring example is the 30-06 improved "can't do anything more than a 30-06", but the .300 WM rules the world. When solutions are worked at and found we find this not to be true. Which is most of what (gun)writers say anyway. I wish that at least if they said something they would back it up with valid (read: true) statements and information. They don't, usually, has been my experience.
As was pointed out the .264 is basically two calibers smaller than the 7mm. Therefore, the 140 gr. bullet is heavier in relation to the bore diameter than the 7mm is. The problem is you are trying to squeeze the same powder/pressure/plasma through a smaller hole. You also have a slightly longer bearing surface and therefore slightly more friction. Although that is almost negligible. Thus it creates more of a backup than does the 7mm. The advantage though is the smaller bullet diameter is more ballistically efficient.[;)][:D]
Tailgunner, I understand what you're saying about MAP versus MIP and I understand the concepts of different powders having different burn rates and temps, but I'm talking the same powder, basically the same case. The only difference being. 020" in bore dia. I never load max loads anyway. My loads are middle of the road at best. I'm just trying to understand the disparity in pressure differences. Thanks for the input. Makes for good conversation.
OK. So a 264 dia bullet of 140 grains will have a longer bearing surface than a 140 grain bullet of 284 dia. So that would explain why(I think)that a 264 would see sharper presssure rises than a 284 dia. That still doesn't explain why the 7mm Mag is loaded to a higher pressure level than the 264 win mag.
Considering the same barrel length, the VOLUME of the barrel for the 7mm Mag is more than the 264. This provided more room for the plasma, and gases to expand, and push the bullet faster. Thus burning the powder more complete. The 7mm Remington Mag is more efficient than the 264 Winchester, and weight for weight, can push the bullet faster...much like a 30-06, and a 270...the 30-06 will ALWAYS be able to push the 150 grain bullet faster.
Back in the day, when the 264 was introduced, we DID NOT have the powders available to us, that we have today. So to try to get the extra velocity we desired, we added case capacity, powder, and increased pressures. Thus is why the 220 Swift, and the 264 were labeled as barrel burners. Much like the 7mmSTW, the 7mm RUM, and the 300 RUM, are today...average barrel life is around 1000 rounds. Be nice, and slightly reduce the loads, don't shoot the barrel, hot, or dirty, and you can extend that. Abuse the barrel, and you can dramatically shorten the barrel life to 500 rounds or less...even with the powders we have today.
This is not saying that the 264 is not a great cartridge, as it sure is. It was designed for hunting at long distance, and for the most part, shooting once a year to check the zero, and killing game....most likely, not much more than a box. It is when we like to shoot a lot, that we discovered the flaws of not having longevity, of barrel life.
So, your concerns of the 264, not holding as much as the 7mm, basically boils down to where they hit the pressure limits that SAAMI, has set for the cartridges...nothing to do with the total capacity of the case.
Best