Optimum Barrel Time

[joeb33050]

"drinks" on the Cast Boolits forum referred me to http://www.the-long-family.com/ and Christopher Long's "Acoustic Shock Waves and the Optimal Barrel Time loading concept!"
I printed the 17 page paper and have read it maybe a dozen times. While I'm still unclear on some of it, this paper presents a predictive model for a set of accuracy zones determined by the optimum barrel times.
If we select one of the array of optmum barrel times corresponding to a node, then fiddle with Quickload to select a load with that barrel time, then that load (or one very close) should be a local accuracy maximum.
Christopher Long's model was derived, if I understand the paper, using relatively high speed jacketed bullets and close-to-full cases of powder.
In all I have read about barrel vibrations and harmonics and nodes (much, I suspect, due to that picture of the vibrating barrel in the Lyman handbook), this is the first predictive model I can remember.
Now, does it work for cast bullets?
To find out, we can approach this thing from the other end. If we list a lot of accurate loads and their associated information, then we can plug that information into Quickload and find the barrel time for each load. We can then compare that barrel time with the array of optimum barrel time nodes, and see if the two barrel times match or are related.
So, will some of you send me your most accurate loads and info?
Here's what I think is needed:

Barrel length (inches, bolt/breech block to muzzle)
Cartridge (Ex: 308 Win.)
Bullet (Ex: Lyman 311041)
Bullet weight (grains)
Bullet length (inches)
Powder (Ex: Unique)
Charge (grains)
OAL (cartridge overall length, inches)

Please send them here or to joeb33050@yahoo.com

Then I'll find some folks with Quickload to figure the barrel time, and compare the numbers to the numbers on the spreadsheet that I made. (I'll be happy to send the EXCEL spreadsheet to anyone who wants it.)

Maybe this will give us a method of predicting and finding accurate loads much faster and less expensively.

Thanks;
joe brennan

[joeb33050]

The first three loads checked had one on the money and two close. How about sending some more of those accurate loads?
Please?
joe brennan

[felix]

Joe, see the post I made about the 257WM to Goose. That will give you some additional feel for what this is all about. ... felix

[Bass Ackward]

Joe,

Take a crystal wine glass and wet the rim and your finger. Play with that for awhile and learn how speed and pressure work together to change the volume and frequency of the sound which is really produced by vibration. Then listen to a consistent pressure and speed which indicates a .... consistent bore condition sounds. This is harmonics 101.

This will give you a better idea of the difference between lead and copper. Because lead will throw less friction than copper (from speed) on the front half of the bullet that is not feeling enough pressure to obturate. But lead vibration occurs from more pressure (obturation) on the base. Still lead overall produces less vibration than copper. Which is why slow powders can cut lead obturation and vibration and alter the whole vibration pattern. Or changing bullet hardness. Or lube.

This little wine glass demonstration will help you understand the role of bullet hardness towards accuracy that you were asking about the other day. It will also start you thinking how lube can come into play to tune the friction end of it. Slipery lube or not? Depends on how it moves you in the node. Which is why a handgunner tends to look at lube as a seal and a rifleman looks at lube as a friction changer. Handgunners seldom consider harmonics really.


Back to copper. Some factory loads work quite well across several rifles. These loads tend to produce consistent accuracy because they produce consistent barrel times and vibration. What happens if your bore condition or vibration pattern doesn't?

Well then you need to taylor your load until it does. That's why so many change powders too. One powder can't do it alone. That's handloading in a nutshell. How many factors go into this? Well, the list is almost endless from how smooth the bore is and how metal fouled it is to what temperature is outside. Even where you are in the life cycle of that barrel makes a difference which is why you need to alter loads slightly over the years.

One friend of mine that helped me when I was young had a 270 that cut holes basically twice a year. Spring and fall. In between there, it was a 1 1/4" performer with that load especially in the summer. He always thought it was him. :grin: We shot almost weekly. So I would bet him when I knew I would win. He could never figure it out and he passed before I ever told him.

So the load information you are asking for is of little use other than to see where it varies from the norm. This is good though, because it has you thinking.

[wiljen]

I read through the paper more than once trying to figure it all out. I think his calculations might only be valid for barrels of that particular make too as I would suspect that changing the alloy of the barrel would also change the harmonics.

His formula for calculating the OTB could also be used to calculate Muzzle velocities of loads that should be the accuracy max assuming linear acceleration (which I know is not true, but it doesn't fall into time-dependent acceleration either). I calculated a few of them just toying around and found that for a 24 inch barrel the first node would be almost 5000fps.