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Relationship between case capacity and propellant type

 
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  #1  
Old 05-02-2001, 10:36 PM
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Join Date: May 2001
Posts: 5,830
Relationship between case capacity and propellant type

Gentlemen,
I'm planning on having a local riflesmith build me what amounts to a knock-off of the Dakota Longbow, only chambered for the 7mm Dakota Magnum cartridge. I own a Longbow and love the hell out it. But the .338 Lapua Magnum is not an appropriate cartridge for taking shots on medium size game at under a half-mile. It's too much power! So I figured I'd have a rifle made just like it, but in 7mm Dakota - a cartridge I feel is one of the most versatile for long range hunting. If I went this route, however, I'd be assembling loads far short of maximum velocities. The reason being that I just don't think one needs to push a 162-grain HPBT projectile to more than 3150 fps to get the job done - even at a half-mile. To be quite honest, in fact, I'm pretty obsessive about this aspect of ballistic performance (i.e. I just don't like any cartridge that produces bullet velocities of less than 2850 fps or more than 3150 fps, whatever the projectile being used.) But I think there would be a problem in my approach. Here's why.
During a telephone conversation with Sgt. Dean Michaelis a couple months back, I learned that a batch of cartridges typically produce the least amount of spread in velocity from one shot to the next when a the propellant that best fills the case within the limits of safe/reasonable pressures is used. This being the case, I thought, its stand to reason that the same propellant would also produce the best, or at least near the best, accuracy. After all, the velocity spread can get pretty significant with some of these big, hot cartridges and it makes a difference in POI when one shot comes out at 3000 fps and then the next comes out at 3050 fps.

If my inference is correct, then can I expect to get good accuracy taking a cartridge like the 7mm Dakota and prepping loads for it that do not produce velocities any higher than we would see with a 7mm Remington given the same projectile? In other words, would I get a good, accurate load for long range that has a whole lot of open space inside the case?

Help me out, fellas!

Dave

N/A

[This message was edited by David P. Herne on April 24, 2001 at 06:47 PM.]

posted April 24, 2001 06:39 PM

Warren Jensen
Member

From: Arco, ID US
Registered: April 12, 2001
Posts: 8
Case Capacity and Propellant
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David,
It will be an interesting project. I have worked with several 7mm Dakota rifles and it is a flexible cartridge with quite a few powders in the proper burn rate to experiment with. Dean is correct in saying that as a rule the most consistent and accurate powders will generally be those that give 100-102% load density for the bullet and seating depth used. There is no real mystery here. The reason that they are more consistent is that the powder position in the case is fixed and the powder does not move around. The ignition and combustion is simply more uniform. You can get accurate loads with load densities below 100%, but there will be more variables.

What I would suggest is if you are looking to find a load that is not utilitzing the full velocity potential of the cartridge, and you want to obtain a targeted velocity, then find a powder that is slightly slower than optimum, say H1000, for that case. That way the efficiency will be down a little , but the case will be full. There are many other powders in this speed range, so you should be able to find one that shoots very well.

Warren@lostriverballistic.com http://www.lostriverballistic.com

posted April 25, 2001 07:40 AM

David P. Herne
Member

From: Houston, Texas (USA)
Registered: April 11, 2001
Posts: 5
Case capacity and propellant type
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Warren,
Perhaps you can help me out on this once I have the rifle completed! I know this happens to be in line with the type of services you provide at L.R.B.T..
But let me mention something else pursuant to this problem. It begs the question: Why even chamber the rifle for the 7mm Dakota if I have no interest in producing velocities with a given projectile that would be any higher than those possible with the 7mm Remington? Why not just chamber it for the latter cartridge and be done with it? Answer: Because the 7mm Dakota cartridge is ostensibly more inherently accurate than the 7mm Remington! This, of course, begs the question in turn: What is it about the 7mm Dakota that makes it inherently more accurate than the 7mm Remington or perhaps any other long range 7mm cartridge? Please see topic on belted versus beltless cases for the discussion.

Regards,
David

N/A

posted April 25, 2001 03:36 PM
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  #2  
Old 05-08-2001, 01:12 PM
 
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Re: Relationship between case capacity and propellant type

Although a relative newcomer, you might consider the 300 Winchester Short Magnum. The US Army marksmanship unit at Fort benning has built a few and they are getting consistent 3100 fps tight groups with raw 175 Match Kings (typically with 10 grains less powder than necessary in a 300 Win Mag). A side-by-side comparison of the short, fat cases make the Short mag and the Lazzeroni 30 Patriot scaled-up clones of the world benchrest standard, the 6PPC. Add the hundreds of different 30 caliber bullets you could spend the rest of your life developing loads for and I think you may have found what you may be fishing for, in a short action, to boot.
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