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Old April 13, 2010, 09:34 AM   #1
M1A1
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velocity effects on grouping

Does more velocity generally give smaller groups? I am trying to tighten up my grouping with with a couple loads and was trying to decide if i should start working up or down.
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Old April 13, 2010, 09:42 AM   #2
Brian Pfleuger
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The timing of the bullet leaving the barrel is what effects groups, so velocity is primarily a factor in how it relates to that timing. Assuming, that is, that the velocity is appropriate for the bullet and twist so that nothing "funky" is happening in that regard.

See Dan Newberry's discussion of "Optimal Barrel Time":

http://optimalchargeweight.embarqspace.com/

You shall now be obsessed.
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Old April 13, 2010, 10:22 AM   #3
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I have found

that when working up a load when my strings look more vertical, I will go up in powder
When they go horizontal, I reduce my load
This is all done while looking at pressure as well.
If the load is not giving the results I want , then I will go to a different powder, bullet, primer, and even case.
Ed
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Old April 13, 2010, 03:31 PM   #4
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Uniform velocity from round to round is often more important that absolute velocity.

The barrel starts vibrating and moving as soon as the primer is struck.
You would like the bullet to exit at the same point in the barrel vibration every time.

Achieving that is the hard part.

If you have a load that is just not grouping well, I always try adjusting bullet jump to the lands before changing powders.
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Old April 13, 2010, 10:47 PM   #5
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I dont think I have ever loaded for speed. It just happens that most of my most accurate loads are right around max. When I'm working up a load, I just watch for pressure signs and the grouping. I usually dont even break out the chrono until I have the group I'll stick with.
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Old April 14, 2010, 01:19 AM   #6
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Quote:
I have found
that when working up a load when my strings look more vertical, I will go up in powder
When they go horizontal, I reduce my load
This is all done while looking at pressure as well.
If the load is not giving the results I want , then I will go to a different powder, bullet, primer, and even case.
Ed
Interesting observation... Does this apply to your loads in general or is it particular caliber ?
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Old April 14, 2010, 08:51 AM   #7
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I was doing some math to see what the optimum speed of a bullet exiting the barrel would be so that the vibrations were at the breach instead of the muzzzle but ran into a snag.
How long would a bull barrel resonate/vibrate with a heavy 25.6 inch barrel?

Thanks, Robert
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Old April 14, 2010, 08:57 AM   #8
Brian Pfleuger
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Quote:
Originally Posted by Hammer1Down
I was doing some math to see what the optimum speed of a bullet exiting the barrel would be so that the vibrations were at the breach instead of the muzzzle but ran into a snag.
How long would a bull barrel resonate/vibrate with a heavy 25.6 inch barrel?

Thanks, Robert
The size of the barrel effects the amplitude of the vibrations. The timing is effected by the length.

Optimal Exit Times would be:

1)0.89920 ms
2)0.98488 ms
3)1.12766 ms
4)1.21394 ms
5)1.35612 ms
6)1.44299 ms
7)1.58458 ms
8)1.67205 ms
9)1.81304 ms
10)1.90111 ms

Pertinent information can be found here:
http://www.the-long-family.com/optim...rel%20time.htm
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Old April 14, 2010, 09:53 AM   #9
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And it needs to be pointed out fast and slow powders don't produce the same barrel time for any given muzzle velocity. That is because faster powders peak their pressure earlier in the bullet travel down the bore. That means a greater portion of the total bullet acceleration takes place earlier in the barrel with a faster powder, giving the bullet more velocity earlier in the barrel length and therefore shortening the total time it takes to get down the barrel. So there isn't a single velocity that will give you best accuracy. Just single barrel times.

You can work with QuickLOAD to predict barrel times correctly for your gun and chamber, but if you don't have the software or feel you're not interested in getting into that level of technical detail, follow Peetzakilla's link and click on the Home page link and read Newberry's whole site. He has a round robin method that is very practical for finding those sweet spots just by evaluating the targets.

As the barrel time list shows, there is more than one sweet spot load for all barrels, but it tends to be the case for most rifles that the shortest sweet spot time (fastest sweet spot load) that doesn't create excessive pressure is the most accurate. At extreme long range you generally get best accuracy by keeping the bullet from dropping into the transonic velocity range before it gets to the target. That means keeping it supersonic with a margin of a couple or three hundred feet per second if you can, which requires a minimum muzzle velocity that depends on the ballistic coefficient of the bullet. Those are the two instances of accuracy being better with more velocity that come to mind.
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Old April 14, 2010, 09:55 AM   #10
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Peetzakilla,Unclenick,

Damn!!!! I am just blown away with the knowledge base on these forums.

I mean that as a compliment.

Geetarman
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Old April 14, 2010, 10:18 AM   #11
Brian Pfleuger
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Quote:
Peetzakilla,Unclenick,

Damn!!!! I am just blown away with the knowledge base on these forums.
If I have any knowledge that's helpful, it all came from TFL (the usual suspects hereon), Dan Newberry and associated sites. I'm just regurgitating it.

I agree, the information here (and there) is mind-boggling. A matter of months ago, I couldn't have told you the parts of a case. At least now I know that the head and mouth are on opposite ends!
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Old April 15, 2010, 12:46 PM   #12
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I to am taken aback by the info and knowledge on this site, folkslike PetzaKilla, Unclenick, and others and of course all the staff have helped me A LOT in becoming a better shooter... for that I thank you guys...

But on to peetzas' post on bullet exit times what are those measured in?

Thanks, Robert
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Old April 15, 2010, 01:18 PM   #13
Brian Pfleuger
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Quote:
But on to peetzas' post on bullet exit times what are those measured in?
Those are measured in milliseconds. Those times begin at the instant of ignition and are basically the points in time when the muzzle of the barrel has the least disturbance.

Follow that link above and/or do a little googling about Dan Newberry and "Optimal Barrel Time". You will have all the reading that you can handle.
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Old April 15, 2010, 03:07 PM   #14
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VLADAN

The different strings of vertical and horizontal patters come from studying fluid dynamics.
Barrels and bullets are not fluid, but the dynamics are the same. The frequency and amplitude are changed by the velocity of a bullet or projectile as it moves down the barrel. The LS-DYNA Code will show that when you increase with pressure at the action the vertical vibration is somewhat damped or mostly eliminated by other factors such as stock magazine cutouts etc, Euler's formula. At lower pressure, vibration is not transmitted into a horizontal vibration. In the 1970 while working at the Rock Island arsenal the guys over at the vibration lab supported my findings. Most of the vibration testing is conducted in a single DUT axis at that time. I have found no rifles with horizontal mounting screws. On howitzers it can be seen for they have the barrel mounted on the horizontal plane. This gives the reverse. Anyone who was in combat and called in the 105 howitzers for support will tell you. They were on target but they would land too close or overshoot the target at times. For max loads the vertical mounting plane is gone which will transmit into a vertical shift.
I can send you the math if you want
give it a try!
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Old April 15, 2010, 03:13 PM   #15
edward5759
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in reading "Optimal Barrel Time"

It gives a better explanation of what I was saying
but put in the accounting for a few more variables.
Ed
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Old April 15, 2010, 03:29 PM   #16
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You can check out Varmint Al's site, if you're not familiar with it? He uses that same FEA package and captured some good animations of the different vibration modes and frequencies and offers some explanatory discussion.

http://www.varmintal.com/amode.htm

http://www.varmintal.com/atune.htm
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Old April 15, 2010, 03:55 PM   #17
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awesome, great reading, thank you gentlemen
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