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Old July 22, 2006, 07:24 PM   #1
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barrel length/ballistics

I am wondering what the formula for ballistics vs barrel length is. For example, what is the difference in power for a .40 s&w bullet fired from a beretta m96 and a cx4 storm. 5 inch barrel vs a 16 in barrel. Is there a simple formula or is it more complicated than that....Thanks I appreciate the help.
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Old July 22, 2006, 08:35 PM   #2
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Nope. No simple formula. It's much more complicated than that. Best way is to find a copy of each and fire a group of the same loading from each and measure velocity. Get an average difference.
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Old July 23, 2006, 03:45 AM   #3
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There are some "internal balistics" programs that do a good job estimating velocity, pressure. stability and so on.

The problem is pretty complex. There are "burn profiles" for different powders. You have to account for how much is burned (and thus, the surface area of exposed powder), the pressure, the temperature, the resistance of the bullet to moving, and probably a few other variables I can't think of at this hour.

Then you have to integrate all these things over time as the powder burns and the bullet goes down the barrel. A simple formula may exist for a single powder, single bullet, single seating depth, single charge weight, and various barrel lengths. But I imagine you want to look at different powders, bullets, and so on....


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Old July 23, 2006, 11:34 AM   #4
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Ditto the above. The basic problem is that powder pressure curves peak early in a bullet's travel down the tube, then drop off from there. All else being constant, how quickly the bullet accelerates depends on that pressure. So, the bullet gains speed at different rates in different parts of the barrel. More quickly in the early inches, less quickly in the later inches. Shortening a tube one inch from 15 inches to 14 inches will cost you a lot less muzzle velocity than shortening it from 4 inches to 3 inches with that same load.

SAMMI specifies standard test barrels, with specific lengths, twist rates, rifling land width, land count, bore and groove diameters and chamber dimensions. So, when you see an ammunition manufacturer post an unqualified velocity claim and no test firearm specification, you can assume it was done in the standard SAMMI test barrel for that chambering. This information, in turn, may or may not correlate well to your gun. If you really want to know what's happening, get a good quality chronograph, like the CED Millenium or a used Oehler 35P (no longer available new).

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Old July 23, 2006, 04:58 PM   #5
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barrel length/performance

One reason that the world changed over to bottle necked cartridges for rifles is that there is a point of diminishing returns as concerns barrel length vs. performance. As noted above, it is a complicated subject, but one that's easy to understand: Friction slows the bullet down enough to begin to affect velocity when the barrel is too long for the powder charge to keep expanding enough to push the bullet viorously.
Where is that point? Hard to say. Somewhere before 16" for a .40 SW, I'm thinking.
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