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 July 22, 2006, 07:24 PM #1 norm582 Junior Member   Join Date: February 1, 2005 Location: Twin Cities, Mn Posts: 12 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.
 July 22, 2006, 08:35 PM #2 Bud Helms Staff   Join Date: December 31, 1999 Location: Middle Georgia Posts: 13,155 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. __________________ "The irony of the Information Age is that it has given new respectability to uninformed opinion." - John Lawton, speaking to the American Association of Broadcast Journalists in 1995
 July 23, 2006, 03:45 AM #3 tINY Senior Member   Join Date: April 26, 2005 Location: Orygun Posts: 2,589 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.... -tINY
 July 23, 2006, 11:34 AM #4 Unclenick Staff   Join Date: March 4, 2005 Location: Ohio Posts: 13,643 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). Nick __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member
 July 23, 2006, 04:58 PM #5 amamnn Senior Member   Join Date: May 13, 2006 Location: WA, the left armpit of the USA Posts: 1,323 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. __________________ "If the enemy is in range, so are you." - Infantry Journal

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