The Firing Line Forums

Go Back   The Firing Line Forums > The Skunkworks > Handloading, Reloading, and Bullet Casting

Thread Tools
Old July 22, 2006, 07:24 PM   #1
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.
norm582 is offline  
Old July 22, 2006, 08:35 PM   #2
Bud Helms
Join Date: December 31, 1999
Location: Middle Georgia
Posts: 13,069
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
Bud Helms is offline  
Old July 23, 2006, 03:45 AM   #3
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 is offline  
Old July 23, 2006, 11:34 AM   #4
Join Date: March 4, 2005
Location: Ohio
Posts: 11,376
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).

Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Benefactor Member
Unclenick is offline  
Old July 23, 2006, 04:58 PM   #5
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
amamnn is offline  

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

All times are GMT -5. The time now is 11:07 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2015, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2015 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent:
Contact Us
Page generated in 0.06321 seconds with 9 queries