OBT barrel length measurement

[Bobr]

What is the general opinion on where to determine the barrel length for OBT purposes. Muzzle to lug,mid chamber.or total length to bolt face.
Thanks Bob

 

[Leaddog]

Muzzle to boltface. There is no other measurement to determine barrel length.

 

[Erik Cortina]

What is the general opinion on where to determine the barrel length for OBT purposes. Muzzle to lug,mid chamber.or total length to bolt face.
Thanks Bob

Charlie beat me to it, but he's right, it's from bolt face to muzzle.

 

[Bobr]

Just to be clear what I am looking for is the length that will be used in determing the optimal barrel time calculations, if these were the correct answers for this thank you.
Bob

 

[Steve Blair]

Just to be clear what I am looking for is the length that will be used in determing the optimal barrel time calculations, if these were the correct answers for this thank you.
Bob

Quoting Chris Long, author of the OBT Theory:
"Barrel lengths are measured from the bolt face to the muzzle crown."

 

[Nomad47]

Excellent, sleepygator.

I might add that Chris' initial tests were with the 223 Remington and calculated from the center of the case, so his formulae are very slightly off with the very long cases. But it only amounts to .001 or .002 milli seconds - not enough to warrant any formula adjusting.
As you said, his calculations are based on the muzzle to bolt face measurement.

 

[Bobr]

Thanks guys

 

[Bulseyetom]

Where would you measure to if you have a muzzle brake?

 

[Steve Blair]

The muzzle crown. The brake means nothing in terms of bore dimension.

 

[Nomad47]

Sleepygator, allow me to run something by you.
On my 284 Winchester which has a Harrell's brake, the kind that screws on and looks like part of the barrel, I found a combination worked best. I used my 28" barrel length in QL to see time in barrel of the bullet, but I used the overall length including brake to come up with an OBT node. It worked. My theory is that you want the time that the bullet exits the muzzle. But the swell that Chris Long speaks of travels to the end of the brake and back.
Your thoughts? Or anybody else that wants to chime in?

 

[Steve Blair]

No, the material discontinuity between the barrel and the brake would prevent that. I've found that the imprecision with which we measure barrel time means that some error must be accounted when using the information. Although I have not used it, the RSI PressureTrace II in conjunction with QuickLOAD is said to give more precise barrel times. [br]
My results with PVM chronograph and QuickLOAD have gone both ways. Even adjusting for true muzzle velocity, Burning Rate Factor (Ba), Shot Start Pressure, etc., the best result has often been a little above or below a calculated OBT node. It is still better than guessing from loading manuals and wasting a bunch of components, time and barrel life trying to find a good combination. [br]
http://www.shootingsoftware.com/index.htm

 

[Steve Blair]

Bill, I am still not 100% convinced that what Chris Long's theory proposes is exactly what is happening. What I do know is that my results show a strong correlation between OBT nodes and consistently accurate loads. I have thought about several approaches to investigate the phenomenon but all of them would require a fair equipment expense and it is not a high priority for me. It is currently enough that I can make reasonably close predictions about what component combinations are likely to produce good results for my long range loads. What all of that means is that it may not serve much purpose to get overly technical about the details. If the way that QuickLOAD seems to work for that particular rifle is to include the brake in the barrel length, then do it. As mentioned before, I have to fiddle QL input in several different ways for different rifles. Once an approach is developed for a given rifle, further predictions are pretty close. [br]
Powder Burn Rate Factor is where I see most variation. H4831 SC seems slower than the QL values in .284 Shehane and faster in 6mm Remington. The values almost always rise slightly as pressure rises. So, the simple answer is that there is no simple answer. When I worked on the Ultra Lightweight Howitzer Program with Picatinny Arsenal, I learned that the Army Ballistics Research Lab (BRL) did not calculate internal ballistics but used empirically derived data to characterize a given charge. They considered it too complex to calculate accurately.

 

[Nomad47]

Very interesting, Steve. Both posts. Here I thought I was on to something with the length combination. And it turns out to just be coincidence.
I adjust four parameters in QL to get things to track; powder burn rate, weighting factor, bullet weight (to simulate an increase in barrel friction), and shot start pressure.
With my particular combination of bullet, primer, barrel, etc in my 284 Win, I had to increase default burning rate for H4831sc by 7%. I don't like to adjust Ba by that much but that was the only way I could get QL to track.
Yes, I definitely agree that QL is not an answer for everything. But it sure can speed up the process and save some barrel life, etc.