Do you feel that OBT loading is worthwhile?

[Dgd6mm]

Look up Chris Long's optimum barrel timing chart. He has done the work for us.
We use it in conjunction with our Quickload program. It cover different barrel lengths were the vibration frequency is at the action when the bullet exits the barrel.
The Quickload program has libraries for bullets, powders, brass H20 capacity and much more. It works in milliseconds. You can change the defaults for your specifics. Knowing as a reloader that the program has certain inputs that will not be identical to your brass H20 or the bullet length could be a little off and the burn rate of the powder could be tweaked if you know it.
The better the information is inserted into the program defaults the closer the results will be with a decent load. My results have given me 1/2" 5 shot groups at 100 yards or better.
From there tweaking a load is fast if you are looking for better accuracy.

 

[ebb]

I think time and technology have left me behind.

 

[fatelvis]

How does one figure out the compression waves traveling back and forth in the barrel?

There is a table that has been constructed showing these times using the barrel length and the period of time the bullet is in the barrel. Truly amazing in my opinion

 

[ebb]

^^^^^^ amassing is an understatement

 

[CF Junkie]

It really is simple. If it wasn't, I couldn't do it.

The reflection speed of the steel in the barrel is key as is precise the barrel length. (See post #13.)
Since the reflection is in the steel of the barrel, not in the air of the bore, the reflection is the same for any caliber.
Calculate the time for the reflection traveling at the speed of the steel for the length of your barrel.
That is the speed for one reflection.
Then multiply it for the number of reflections from 1 to maybe 16 or so.
The reflection will be at the chamber for all the even reflections.

You probably will be most interested in the 10th, 12th or 14th reflection, depending upon the velocities of the powder bullet combination you are using. One of those will probably be the Optimum Bullet Time for your barrel - you just need to be sure you watch the chamber pressure to keep under PMax.

Tools like QuickLOAD will give you the muzzle exit time for your load.
Adjust your load, seating depth, etc, to match the bullet exit time to the time of one of the reflections closest to your load and you are at your Optimum Bullet Time.

 

[ebb]

Does the weight of the barrel (wall thickness) have anything to do with it? The barrel could be a #1 taper or a 1 1/2 inch bull barrel and the program works the same for both? Or does the program account for various barrel weights?

 

[jelenko]

It's the speed of sound in steel - doesn't matter the thickness.
Long's theory is that the explosion from the round igniting causes a compression wave [compression means the steel itself is compressing ever so slightly] to travel down the barrel. When this wave gets to the muzzle, it reflects and travels back towards the breech. When it gets to the breech, due to the interface between the muzzle and action, it reflects and travels back towards the muzzle.
Of course when the wave hits the breech/action interface, some of the energy in the wave goes into the action. Long didn't address this but, presumably, his assumption was the wave that would be reflected back from the rear of the action would be very small compared to the primary wave in the barrel.

 

[Murray Brook]

There is a table that has been constructed showing these times using the barrel length and the period of time the bullet is in the barrel. Truly amazing in my opinion

 

[CF Junkie]

In order to even consider that table, you would need to know what barrel steel it was created for.
Also the 'node' terminology doesn't match a reflection and what the values shown represent is puzzling.

For example, for a 24-inch 0.3% barrel steel rifle barrel will take 0.10467 msec to get from chamber to muzzle.
It will take another 0.10467 msec to get back to the chamber so one reflection takes 0.2093 msec.
The fourth reflection - back at the chamber again is twice that - 0.4187 msec.
The 12th reflection would be 1.256 msec.
I have three rifles with 24-inch barrels in two calibers that use that exact OBT.
Unless I am shooting really light bullets, trying to load for the 10th reflection generally gets my loads over PMax.

If the 24-inch barrel steel were 416R stainless steel, the fourth reflection would be 0.3997 msec. and the 12th reflection would be 1.199 msec. I have three rifle barrels made of 416R Stainless with barrel lengths of 18-inches, 26.125-inches and 28-inches, in three different calibers. Their 12th reflection OBTs are 0.897, 1.305 and 1.339 msec. respectively.

Depending upon the bullet weight and powder charge and of course, barrel length, most bullets exit a muzzle between 0.900 msec for an 18-inch barrel to 1.650 msec for a really long barrel (28-inches) loaded with a light charge.
The barrel OBT is based on steel characteristics and length.
The bullet exit time is controlled by bullet weight and length, powder charge, seating depth, and barrel length. The bullet doesn't care about barrel steel, it just reacts to the pressure produced by the powder burn. The powder type and charge determines how the burn rate produces pressure and how the pressure accelerates the bullet down the barrel. Each powder has different characteristics so that changes the rate of burning and thus the pressure and how it builds and how long it is sustained.

I have no idea how the nodes listed in the table in post #28 are even related to OBT and the differences in 'nodes' don't seem to have any reasonable reference to increments related to barrel steel characteristics.
So I would be careful about trying to make use of it.

 

[Murray Brook]

This is where it came from ( Chris Long). Works for me reasonably well.

Techshooters Shooting Pages

 

[Dgd6mm]

This is where it came from ( Chris Long). Works for me reasonably well.

Techshooters Shooting Pages

Absolutely!

 

[jelenko]

Just to confirm.
GRT has a built in OBT 'calculator'.
Once you enter the specifics about your load, barrel length and observed velocities for a sample of rounds, you can use the OBT calculator to determine the powder charge to be on one of the nodes.