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

[green 788]

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

 

[bailey1474]

[ 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.

 

[Guest]

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

 

[Bart B]

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.

 

[BountyHunter]

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

 

[green 788]

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. /ubbthreads/images/graemlins/wink.gif

Dan

 

[Bart B]

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.

 

[BountyHunter]

Bart

Hard to believe, not really.

Excellant groups, but not up to BR standards. I shoot at quantico and seen too many highpower shooters bring on the line and they do ok but not win.

Look at the match reports for 600 yd BR, in the 1-2.5" (aggs, not one group under testing conditions)range all day. You have to shoot the 4" aggs at 1000 to win now under match conditions. they do much better under the low wind early morning/late evening conditions we all test in and report on. Most LR guns shoot in the .1-.2 range and the best shoot in the .0-.1

Bottom line do not use a chrono if you do not believe in it but most people do.

PS Danny Brooks set an IBS 10 match agg (10 shots each) record for the season of under 7.4", all under match conditions and he uses a chrono. Think you will find David Tubbs uses one also.

 

[green 788]

BH,

For the record, I do use a chronograph to give me a good idea of velocity and energy downrange. I'm not advocating not using a chronograph, though I will say that it is possible (and actually preferrable in my opinion) to develop a great load without a chronograph.

In the end, there are various ways to skin this cat. I like to advocate the simplest and often the most modest of tools and methods, as this allows for more folks to become involved in the art (and science) of long range riflery--which is something that I believe should be more common in our country...

Dan

 

[BountyHunter]

Dan

You and I have had this discussion before. OCW is a good sytems for what it does, but it is not good at reaching the upper levels of accuracy.

I can fire 20-30 shots on a ladder and chrono and ID from 2-4 tuning nodes on my barrel. OCW cannot tell you if you have 1 or 10. I know where my pressure is going to jump normally, OCW cannot tell me that. After that it is a simple matter of picking the MV closest to my desired MV, (OCW cannot find that) and then tweaking minor powder adjustments in the middle of the node so I do not worry normally about pressure spikes from temp etc, neck tension and seating depth to get a load that shoots in the .2 and under. We all agree LR testing is required no matter what methodoligy you use.

I and most LR BR/LR shooters do not use the OCW methodoligy because it does not give us the accuracy levels we need w/o firing bunches of groups. It gives acceptable accuracy (.5 moa maybe even a little better on a good day) levels in most guns very quickly, but not the extreme accuracy levels (.0-.2). I can pick any components and get that, so OCW really will not tell me anything.

True most people and any factory gun should be happy with .5 MOA. I am with mine, but not LR BR or custom LR hunting rigs.

If someone is happy with .5, then by all means do it OCW and w/o chrono and be happy. If not, then suggest they try a ladder with a good chrono.

Until 1k BR/LR shooters starting using the OCW routinely, it seems that it is only a theory that it will work with extreme accuracy results. But best I can tell, that theory is not close to being proven at this time.

BH

 

[Eaglet]

Bill Bailey,
It shocks me too; I had never thought that the way you hold the rifle has anything to do with MV. Thinking about it, I read some place that most of the velocities given for factory ammo. is higher than what most of us chronograph due to the fact that factory folks use some kind of a rest and not human shoulders. /ubbthreads/images/graemlins/laugh.gif Don't know how true that is. I would like to think that some of that internal energy is used to push the rifle backwards and if there is no resistance from a firm shoulder then the bullet will loose speed because of the sharing of energy which is, as we all know, what pushes the bullet out. But, in my mind, the other scenario is… it doesn't matter how much the rifle recoils, or resistance to recoil it has, the internal pressures would still be there unless that bullet comes out; by the rifle free recoiling or not does not, in my mind, relief the internal pressures inside the chamber, so it should not affect it at all. Never the less, something tells me I'm not seeing the whole picture! Anyone else cares to comment on this?
Thanks… Good posts!
Javier Moncada.

 

[Bart B]

I'm talking 20-shots (or more) per group. How many are you talking about?

Guess I'll have to find the pictures of some sub 1-inch 10 shot groups fired from a Winchester 70 in 1971(?). The NRA's Rifleman Magazine (same year) had an ad with them in it.

 

[Bart B]

On the same day with the same rifle and ammo, I got lower muzzle velocities shooting offhand, medium shooting sitting and highest shooting prone. Averages were only 10 to 15 fps apart, but the chrono showed that.

I've got as much as 50 fps spread from a 13-pound .308 Win. shooting 168 grain pills off a bench; alternating between one held loose barely touching my shoulder and then pulled hard into my shoulder.

A couple of friends shot the same rifle off sandbags atop a bench back in the '70s and got 102 fps average muzzle velocity difference between them. When they clamped the rifle in an 80-pound machine rest, the rifle shot the same ammo a bit faster and the muzzle velocity spread was the lowest of all due to very repeatable resistance to the rifle's recoil.

Us humans are just not as repeatable as we think we are. Don't forget that if the energy from the powder's not pushing the rifle backwards, all that energy pushes the bullet out the barrel. More backwards rifle movement means less forward bullet movement. Grade school physics.

 

[bailey1474]

Just b/c the rifle doesn't move doesn't mean that no energy was transferred backward. The same energy was transferred, there was just something there strong enough to absorb it and not move. The energy is distributed it's just that no work was done b/c the rifle did not move.

That can be compared to a person pushing on a wall. Just b/c the wall doesn't move doesn't mean that the person did not expell any energy. Energy is expelled but no work is done b/c nothing moved.

I'm still not buying this 100 fps ting. Energy is expelled the same every time throughout the rifle regardless of how the person shooting is holding it.

Maybe I'm wrong, but I don't think so. I know there is someone on here who has a little more detailed background in physics. If they would chime in, that would be great.