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Using chronograph data to determine the best load????

 
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  #15  
Old 12-28-2005, 08:49 AM
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Re: Using chronograph data to determine the best load????

I'm not sure what the actual velocity difference would be, but yes, if one guy free recoils a rifle and another holds it firm against his shoulder, there will be a difference in the distribution of the cartridge's energy. In the free recoil situation, some of the energy of the burning powder is going into pushing the rifle backwards, and this leaves less energy to propel the bullet forward.

Try this next time you use your chronograph. Shoot a few free recoil (if possible) or with a light hold versus firm. You'll almost certainly see some different numbers...

Dan
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  #16  
Old 12-28-2005, 09:43 AM
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Re: Using chronograph data to determine the best load????

[ QUOTE ]
I'm not sure what the actual velocity difference would be, but yes, if one guy free recoils a rifle and another holds it firm against his shoulder, there will be a difference in the distribution of the cartridge's energy. In the free recoil situation, some of the energy of the burning powder is going into pushing the rifle backwards, and this leaves less energy to propel the bullet forward.


[/ QUOTE ]

I'm not trying to be a smart ***** or anything here, but this is just not true. The energy is distributed the same everytime. It's just a matter of whether or not there is a shoulder there to absorb the rearward motion. It's a matter of physics. The energy is the same everytime.

I think the reason you guys are saying that the velocity will change is due to the magnum effect exhibited when you free recoil large caliber rifles. A person who shoots a large caliber rifle free recoil will shoot higher @ long range due to the muzzle jump. Someone who holds hard against the rifle will absorb some of this energy and get less muzzle jump, causing a lower POI. It has nothing to do w/more or less velocity.

Just my thoughts.

B.J.
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  #17  
Old 12-28-2005, 10:31 AM
 
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Re: Using chronograph data to determine the best load????

Bart
In my limited experence with a 6mmBR improved in a Lilja barrel that when I got close to really good numbers ES& SD the gun shot all in the same group. If you have a really good barrel it does'nt matter. look at bench rest shooters who are clicking up or down to adjust for condtions with there fancy powder measures.With factory barrels I have experenced that the low numbers are a little more revelant.
old bear
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  #18  
Old 12-28-2005, 11:05 AM
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Re: Using chronograph data to determine the best load????

BountyHunter....If a chrono is important, then why did half a dozen of folks with some of the most accurate rifles on this planet shooting the same bullet, case and primer get the most uniform muzzle velocity and worst accuracy with AA2520 ball powder? (see my earlier post).

And why did IMR4895 powder producing just average muzzle velocity spreads produce the best accuracy? A couple hundred people shot that ammo in all kinds of barrels with different bore, groove and chamber dimensions; it was the best they'd ever shot.
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  #19  
Old 12-28-2005, 11:39 AM
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Re: Using chronograph data to determine the best load????

The most accurate rifles on "this planet" are BR rifles both SR and LR unquestionably. No one uses AA2520. So no reason to even consider it as a possible powder for either, so not sure where how you define the most accurate rifles.

However, when considering a possible powder a simple ladder with chrono will show you that barrel's nodes with that barrel, bullet, case primer combo. The ladder will show you the grouping node and the chrono will show you the MV node which normally are the same, not always but normally. Easy to pick a powder charge in the middle of the node and then tune seating depth and neck tension.

That methodoligy has been proven time and time again by the some of the best guys shooting the most accurate guns and then some just use the trial and error method, but most still use a chrono for the LR guns. MV, ES and SD are not as important for the SR guys.

As far as your IMR4895, you must be talking for NRA highpower and NRA LR, where the accuracy standards required are much less than LR BR. Have not seen IMR4895 on any equip list for LR BR matches.

Bottom line if your accuracy standard can be reached without a chrono, good for you. However, if someone is wanting to reach the top levels of extreme accuracy, then a chrono has been proven to be a very useful tool for most people who compete in that area.

Kind of hard to imagine spending $2000-4000 on custom guns and not spending $200 for a good chrono to set it up, but to each his own.

BH
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  #20  
Old 12-28-2005, 11:50 AM
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Re: Using chronograph data to determine the best load????

Here is why it is possible to have a tight ES shoot poorly, and a wider ES shoot fine...

A sketch from my archives...



When the main shock wave (the wave which is initiated by the ignition of the powder, and when then travels at approximately 18,000 fps through the barrel to the muzzle, then back, then forth, etc.) has reverberated away from the muzzle, there is still a pattern of vibration at the muzzle. The muzzle is "drawing" a pattern in the air while the bullet is still travelling toward the muzzle to exit the bore.

If the muzzle is moving through a "straightaway" of this pattern, you can actually have a tight ES and the bullets will "string" on the target.

Conversely, if the muzzle is at one end or the other of this vibration or "whip" pattern, it (the muzzle) will be slowing down and nearly stopping before it reverses direction. There is a time window here which will allow bullets being released all along it to be essentially in the same point in space as they begin their flight.

So, you could actually have an ES of 25 fps with, say, a 175 grain Sierra Matchking from a .308 win, and there would be less than 1 MOA of space between the 2625 fps and the 2650 fps shots at 1000 yards--provided that these two bullets were released on an optimal point along the vibration pattern.

And of course you could have an even tighter ES, say 10 fps--and if these bullets were released along the "straightaway" as shown in the sketch above, then they're going to have a more angular relationship to one another, and print farther apart at 1000 yards--even though the ES was smaller.

I do agree that as we approach the optimal load zone, the chronograph numbers tend to tighten up. But that doesn't ensure that we have the bullets exiting the muzzle on a good (near static) node of the vibration pattern.

Which is, to repeat myself, why the target is always the final arbiter. Otherwise, we could save some target paper, and let those guys working the pits sit back here with us while we all see who can get their Oehler 35's (soon to be out of production, by the way) to spit out the purdiest string of numbers for the trophy. [img]/ubbthreads/images/graemlins/wink.gif[/img]

Dan
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  #21  
Old 12-28-2005, 11:59 AM
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Re: Using chronograph data to determine the best load????

BountyHunter, your comment:

"As far as your IMR4895, you must be talking for NRA highpower and NRA LR, where the accuracy standards required are much less than LR BR. Have not seen IMR4895 on any equip list for LR BR matches."

Consider the fact that the best NRA highpower match rifles shoot in the ones and twos at 100 yards just like the best benchrest rifles do. And they'll shoot in the threes and fours at 600 yards (how else could Corky Tyson shoot 20 consecutive shots with iron sights into 4 inches at 600 yards slung up in the prone position?). And fives to sevens at 1000 yards. Highpower match rifles using box magazine actions are equal in the accuracy department along with benchrest rifles. And they do it with full-length sized cases,too.

Hard to believe? Yes, for most folks. But it's reality.
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