Gordon's reloading tool and optimum barrel time values.

[charlieboy-shooter]

Hi Guys,

Recently downloaded Gordon's reloading tool. Still playing with but have noticed that I get either, no value or 3 1/2 or 4 for the optimum barrel time in the results ( think example was .222rem)

Does any one know what this value relates to and really means ? Is it a relative to the node value from Quick load ??
I.e a load with no value- not ideal. 1/2 value ?? And high or low number better?

Thanks

 

[borbal]

As far as OBT is concerned, there is absolutely no scientific basis for this theory whatsoever. Yes, it is relative to the node value in QL, but as the barrel time in QL is going to be different than GRT, the nodes will occur at different loads and so what do you conclude then?

 

[charlieboy-shooter]

As far as OBT is concerned, there is absolutely no scientific basis for this theory whatsoever. Yes, it is relative to the node value in QL, but as the barrel time in QL is going to be different than GRT, the nodes will occur at different loads and so what do you conclude then?

Hi,
I didn't know there was not any scientific basis for the theory. I can't remember who the author was but I have read a interesting and very convincing article and come across the term often so assume there is some merit in it. Although many years ago when I was an apprentice a Forman told me Never Assume anything as it makes an Ass out of U and Me.

My reference to to Quick load, nodes and OBT of GRT was not as a straight comparison. But is the principle meaning the same in its own right for each software package ?
I am to try and understand it's meaning in GRT relative to my basic understanding of the results from Quick load. I have been helped out with some Quick load data with references made to accuracy nodes given as a value which seems to increase as the velocity increases and these nodes are like to provide a more accurate load. Therefore I was hoping that the values given in GRT might indicate just that A load with a value are 'Likely' to be more accurate due to the variables entered into the program than a load without a value.
The fact that a term and value is given in there relative packages must mean something ?

 

[Davee]

. This explains the 1/2 settings.
OBT is a theoretical solution to a complex problem compounded by the fact that they are 3 dimensional in character, barrel vibrations are subject to a multitude of variables, ie metallurgy, length, bore, profile, jump to lands, etc. The factor with the most influence is barrel length, this factor is easily changed merely by adding weight, ie a moderator, muzzle brake, chronograph etc. Barrel tuners exist in many forms, but all involve changing the forces applied to the barrel.
OBT is not a 'be all, end all' solution but provides an indication of where a stable load may be found. It will be different for every barrel, but with prolonged observation you can determine where the nodes for your barrel are. Even the bullet jump will change it, and a stable load can often be improved by adjusting the jump.
The use of OBT is more prevalent in long range shooting circles than in a normal hunting scenario, but why not take advantage of anything that improves the consistency of your reloads?

 

[borbal]

Well, here is Chris Long's original paper. The idea is that the pressure pulse in the chamber causes an increase in the barrel diameter around the chamber. This creates radial vibrations that propagate down the barrel to the muzzle. It is claimed that this causes the bore diameter at the muzzle to increase (and decrease) by up to 0.0002". It is posited that this can affect accuracy if the bullet arrives at the same time as a node in the radial wave, at which time the bore diameter is varying rapidly. It is claimed that good accuracy is obtained if the load is varied such that the bullet arrives at the muzzle when there is an antinode in the radial pressure wave and the diameter is not changing with time, preferably a negative antinode when the barrel is tight rather than loose.

There is no actual experimental evidence where these radial waves have actually been observed, or that the bore diameter at the muzzle changes in any way before the bullet arrives there. It is an easy enough experiment to do these days.

Chris Long invented this piece of pseudo-science to try and explain why a group with (say) 31 grains of powder will be all over the target, whereas 32 grains of powder will be all in the same hole, and 33 grains of powder will be all over the target again. To the extent that this is genuine phenomenological effect, rather than simply poor statistics, I would prefer to see an explanation that was grounded in physics that we know about...

 

[Davee]

I can envisage that the bore could change behind the bullet as that is where the high pressure/high temperature zone is, how this would affect the bore in front of the bullet I would not hazard a guess and will definitely pass on that one. As you say technology has improved by leaps and bounds since he postulated that theory. I am sure that a set of piezo-electric crystals could be used to measure any changes, whether it would be financially viable is another matter. As the bullet passes down the barrel , the barrel is compressing it, there must be an equal effect of the bullet on the barrel. The fact that the barrel vibrates has been shown using high speed photography, and that the vibrations are not simple but are groups of harmonics in all planes is evident. Maybe we should look for a Euro-millions winner who is also a shooter to sponsor the research, after all what is 3 million quid when you've just won 350 of them!!
I cannot see a logical explanation as to why a barrel tuner would or could affect the change in bore theory, a thicker barrel maybe. Weights can affect the POI as otherwise how does that change when your take the moderator, muzzle brake, dampener, chrono, etc. off? I can understand how these affect other vibrations caused by the firing process. Is it just co-incidence that all of my most precise loads seem to coincide closely to a node, never spot on but relatively close -always with a moderator which the bore theory does not take into account- and slightly different with each type of bullet?
I suspect that there are a great number of variables involved, some of which we haven't identified, at play here and I doubt that they could all be eliminated with just one solution. The harder we look the more we will find.

 

[borbal]

Piezoelectric transducers to measure pressures inside the barrel indeed cost several thousand pounds each, but strain gauges - such as are used in the Oehler device - cost just a few pounds each and are exactly what is needed to see if there is any Chris Long effect. As one might expect, there is a vast amount of data 'out there' showing chamber pressures measured with strain gauges and some data with strain gauges on other places on the barrel. If the Chris Long effect existed as a real phenomenon, we would know about it.

But there are many ways in which a barrel can vibrate. There are radial vibrations, torsional vibrations, longitudinal vibrations and transverse vibrations. The last of these has the most impact on accuracy, as you can see in this article of mine.