O B T optimum barrel time ?

[Tripodmvr]

AI Copilot has the following negative comment on the OBT method.

Complexity: OBT requires precise measurements and calculations, which can be time-consuming and challenging for beginners.
Barrel Wear: Frequent load development using OBT may accelerate barrel wear due to increased shot count during testing.
Limited Application: OBT is primarily useful for precision shooting; it may not be as relevant for other shooting disciplines.

[223]

A few observations:
There are many variables at play in a barrel when the primer pops.  Most of them are not accounted for in the OBT calculators.  Things like bullet/barrel friction, bore diameter variations, jacket fouling and many more.
So, all else being equal, a 150gn PMP bullet and a 150gn Impala (turned brass) bullet exiting at the same time (0.150ms) may or may not travel at the same velocity.  I specifically used these 2 examples, as their construction and materials differ a lot. The pressure curve may also differ, resulting in different acceleration curves.
Your best bet is to do the whole calc again with any new bullet.  The final calibration from actually measured MV data will give you the correct estimated OBT and load.  Some may be similar, others not.

[Tripodmvr]

I am a happy chappie when it comes to Quick Load and OBT. From about 1500 calculations for many users I have achieved a success rate of 95%. I am therefore a firm believer and know that it works, even if the speed and barrel time differ slightly. The width of the accuracy band insures that.

[Tripodmvr]

Attached are three theoretical calculations to show @Treeman what changes in bullets might have. Distance from the lands, burning rate of your specific lot of propellant, true barrel diameter and other factors might affect the charge and speed initially achieved. It seems however if speed might be fairly constant for cup and core bullets to get on the node. Reports of a speed of 2950fps for the 150gr bullets in a 270W then seems to ring true.

Due to the large engraving force with the 150TTSX node 4 might not be a good idea.

[Tripodmvr]

Thought it relevant to use a slower burning propellant with the TTSX. The node speed has now increased substantially and pressure is down. This is due to the higher acceleration further down the barrel. Look at initial time to reach max pressure.

[Treeman]

Very interesting stuff,sorry for my silence, I was in a very bad place 
https://allgunstuff.co.za/index.php?topic=768.0

[Againstthegrains]

Ok, I still do not get it !
If 0.149 is a accurate node for a 130 grain bullet in a 22 inch barrel, and 0.149 is achieved by 2500 fps, surely then a 150 gr bullet launched at same speed will achieve same barrel time which then means its on and node, same node as any bullet at that velocity exactly leaving that barrel in that exact time 0.149 ms

WHOT ! have I got wrong ?

You are right, in theory that is correct, give or take a few ft/sec either side. However, there will be caveats, such as differences in case fill/load ratio. eg. a 130gr bullet in a 270 win fired at 2500 will not have as good a case fill ratio, and will in theory be less accurate than a 150gr. Also seating depth will also play a role.

[Treeman]

Ok, I still do not get it !
If 0.149 is a accurate node for a 130 grain bullet in a 22 inch barrel, and 0.149 is achieved by 2500 fps, surely then a 150 gr bullet launched at same speed will achieve same barrel time which then means its on and node, same node as any bullet at that velocity exactly leaving that barrel in that exact time 0.149 ms
WHOT ! have I got wrong ?

You are right, in theory that is correct, give or take a few ft/sec either side. However, there will be caveats, such as differences in case fill/load ratio. eg. a 130gr bullet in a 270 win fired at 2500 will not have as good a case fill ratio, and will in theory be less accurate than a 150gr. Also seating depth will also play a role.

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The part I am not getting.
It is accurate if it takes .149 ms with a sierra bullet. Surely it is accurate with a Hornady bullet at .149ms as well then? Does not matter how it achieves or powder burn or whatever, sure if it leaves barrel at same time it must be as accurate as the other bullet at same time ?
The OBT needs do not ask what propellant or what bullet, ONLY barrel length. I would then deduct that it is only concerned with time over distance ? This would mean that not knowing your bullet or propellant it still predicts the OBT - Quick load predicts velocity, pressure etc and delivers these numbers which include barrel time - barrel time is then used to find OBT on chart.
I MUST therefor be able to THAT OBT for any bullet that exits that length barrel in that time over distance.

[Tripodmvr]

OBT values are determined by the barrel length and are fixed for that length. This is because the shock wave travels at a known speed in steel.

QL does a calculation and eventually the speed achieved is determined by the gun data (case volume, COL, case length)  and the projectile (diameter, length and friction coefficient). These variables will affect the bullet time in the barrel and slight differences will then understandably be experienced. The components thus play their role in the final barrel time achieved.

[Treeman]

OBT values are determined by the barrel length and are fixed for that length. This is because the shock wave travels at a known speed in steel.

QL does a calculation and eventually the speed achieved is determined by the gun data (case volume, COL, case length)  and the projectile (diameter, length and friction coefficient). These variables will affect the bullet time in the barrel and slight differences will then understandably be experienced. The components thus play their role in the final barrel time achieved.

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Then surely ! Another bullet and propellant if it achieves that same barrel time will be as accurate as any other bullet AT THAT OBT.
I am suggesting that you could load any bullet to a known barrel time using a known needed velocity and loading to that velocity to achieve that barrel time.

[Againstthegrains]

OBT values are determined by the barrel length and are fixed for that length. This is because the shock wave travels at a known speed in steel.

QL does a calculation and eventually the speed achieved is determined by the gun data (case volume, COL, case length)  and the projectile (diameter, length and friction coefficient). These variables will affect the bullet time in the barrel and slight differences will then understandably be experienced. The components thus play their role in the final barrel time achieved.

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Then surely ! Another bullet and propellant if it achieves that same barrel time will be as accurate as any other bullet AT THAT OBT.
I am suggesting that you could load any bullet to a known barrel time using a known needed velocity and loading to that velocity to achieve that barrel time.

Chances of that happening are good, but inconsistent bullets give inconsistent results, irrespective of OBT theory.