O B T optimum barrel time ?

[Treeman]

Not so clever me I am not. Just put that there so nothing else from me seems so doff

Ok! So I load up a load and its accurate, bullet A and powder XY in rifle #1, the accurate load has a barrel time of 0.150 ms.
If I were to load same everything, but use a different bullet, then shoot loads over chroni till I have same velocity, I would have same barrel time ? Would this load then be accurate as its on a node. IE, know what speed = what nodes and just load aimed at those speeds.
 I could then just load a few rounds and find correct velocity ?
I could then use any combo as long as I have a velocity to aim for ?

Is this essentially correct ?
Could we just use velocity/Therefor OBT as an accuracy indicator.
Shoot record velocity - come home, read charts, find correct load/velocity, replicate charge and load up 300 rounds

[big5ifty]

If that were true, then every barrel would have one velocity where all the loads would be equally accurate.

[janfred]

The actual barrel time is governed by the in-bore acceleration of the bullet. Different powders, bullet weights and bullet shapes have different acceleration profiles.

I have no way to measure that so the above is a theory that I have heard from people smarter than me.  I have found that different bullets of the same weight does not perform similarly at the same velocity in my .308 target rifles.

[Againstthegrains]

It depends on how you are splitting hairs? On a .308 Win, your nodes are about 200ft/second apart. If you shoot a 150gr bullet, your best node is at 2845ft/sec and the next at 2615ft/sec, with my 26inch barrel. A faster node at 3050ft/sec cannot be reached because of too much pressure.

Now after trying a bunch of bullets, if I start my load development and I land up smack bang between nodes e.g 2700ft/sec, generally the rifle groups at its worst which is about 2-3 MOA, and as I approach the node, about 30-40ft/sec either side, the group shrinks to sub MOA. Now some bullets I can get to 1 MOA, some down to 1/2 MOA and some slightly better. With all  150gr bullets I have tried, the sweet spot will vary about 30 ft/sec around the node, and with 165gr bullets, the sweet spot is around 2615ft/sec is just as predictable.

On my .308 this all works like clockwork, and if you give me a 150gr bullet and it groups worse than 1MOA at  2845ft/sec or a 165gr at 2615ft/sec or there about, I'd generally say its a shitty batch of bullets. Try weigh the bullets in the batch, and measure the length, diameter and concentricity. You generally find that inconsistent/variable bullets don't shoot as well.

On the other side of the spectrum, my thin 20 inch barrel .204 Ruger doesn't give two hoots about OBT theory. The only time it groups well, is when you load it very close to maximum pressure, and then that velocity doesn't seem to correspond to a node at all. Its a bit unsettling that the moment you feel a slight pitting of the firing pin crater on the primer surface, but before the primers start to flatten or the bolt gets stiff, that is where it groups like a king.

So go figure??? Are the models better developed for medium calibers and therefore OBT theory fits better, or is the behavior of long/heavy barrels more predictable?

So going back to the original discussion, if I try a 150gr bullet in my .308 and it groups best 40ft/sec either side of a predicted node, then I'm not too fussed, as I think this is still close enough to be considered on the node, and consistent with OBT theory, but if a bullet groups best smack bang in between predicted nodes, I'd be somewhat surprised.

I'd love to know what others have experienced with respect to how their rifles "obey" OBT theory.

[big5ifty]

I compared what the software says about loads that I see to be accurate on paper.

The software OBT and actual barrel times are close. This is without me measuring my case capacity.

I do rely on the software to tell me what the expected % powder burn is going to be.

Otherwise I don't get too involved with calculations, I much prefer to go by what I see on paper, even if it costs me the extra 16 rounds for a OCW test.

I won't make a change to the load based on what the software says, only on what the target shows.

I keep coming back to the same statement : OBT is the math behind OCW, and no mathematical model is perfectly accurate for all cases. OCW is accurate in all cases.

[janfred]

People should not conflate the two theories. OBT is not OCW.

Optimal Barrel Time (OBT) is the theory and method to predict which load gives the best muzzle velocity ES/SD by predicting when the annular wave is not at the muzzle. The wave is caused by the sudden increase in pressure due to powder ignition. The wave bounces between the muzzle and chamber end at the rate of the speed of sound in steel. I.e. the muzzle diameter increase and decrease with the passing of the wave. It is not unheard of to have large groups even with 15 round SD of 6fps.

Optimal Charge Weight (OCW) is the theory on why certain factory loads work well with almost any rifle. According to the theory barrels vibrate up and down during the shot cycle. The idea is to get the bullet to exit the muzzle when the angular rate of change of the muzzle is small thereby reducing dispersion. Quite common to have small groups with large velocity SD's.

OBT - small SD, important for long range shooting.

OCW - small groups, important for hunting.

[Treeman]

If the bullet leaves the barrel at .149 ms accurately, surely all bullets leaving at that time will be in the same position at that time.
The OBT does not ask for bullet or propellant, only velocity and barrel length.
Different bullets build different pressures - yes, different propellants create different pressures - yes, but time is time ?

[Tripodmvr]

Initially QL predictions are just that. Once you have all the rifle data correct and then add speed and charge, you can then calibrate your burning rate to get the speed to correspond. Bullet weight, friction and other factors have now been incorporated and you can then fairly accurately use charge adjustments to get the barrel time to agree with the calculated OBT value for the barrel length. You then have to shoot that again to confirm your theoretical calc. If there is a large change in charge weight, then an increase or decrease in pressure might affect the burning rate. Small propellant changes might then be necessary. Going up and down by 0,5gr will then show if you are on the node and how wide it is.

Speed does not always increase when the node is reached. This reflects in OCW as shots that have a small vertical displacement. OCW and OBT are then closely linked.

[big5ifty]

People should not conflate the two theories. OBT is not OCW.

According to the creator, the math was developed to explain why OCW works.

Shock Wave Theory.

First paragraph.

[Treeman]

People should not conflate the two theories. OBT is not OCW.

According to the creator, the math was developed to explain why OCW works.
Shock Wave Theory.
First paragraph.

**************************
This OCW method, dunno, just seems so much premium ammo sent down range and then you only know that tin of powders story.

[janfred]

TLDR: Chris states that the bending moment theory cannot be true because the harmonic frequency by individual barrels are not fast enough to cause the effects noted.



If you read a bit further you will find where Chris also states that

"The singular thing that I could not get past was that a simple harmonic vibration pattern could not explain the fact that a single load could work with so many different rifles."

Also

"The first theory is the possibility that the bending modes of the barrel are getting excited differently each shot. Upon further thought, I concluded that this is not possible, based on the following reasoning. The basic resonant (beam bending or "ruler on the edge of the desk") mode of a cantilevered barrel (beam) made of steel at our typical lengths and diameters is on the order of 500 to 1000 Hz, give or take. Even if you look at higher order modes, such as the 5th harmonic mode, you are still between 2500 to 5000 Hz. Keep in mind these modes are in reality definitely NOT simple beam bending modes, but have a lot of twisting and elongation going on at the same time as well, a very complex situation. At 5000 Hz, the muzzle (assuming it was at a point of maximum movement) would make one cycle every 1/5000 or 0.2 mS. If the dispersion was due to these beam bending modes, then it should be fairly insensitive to small changes in charge and seating depth. Also, changing the barrel mass distribution even a little will completely change the node position, and therefore make it impossible for a universal OCW load to exist. Therefore, I concluded that the sensitivity to small changes was not from changes in the bending modes."

[big5ifty]

This OCW method, dunno, just seems so much premium ammo sent down range and then you only know that tin of powders story.

... for that tin of powder, that bullet, that primer.

[big5ifty]

The originator of the OCW theory also admits he has no idea why OCW is true and repeatable.

[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 ?

[big5ifty]

Why not test it and record the results.

Sometimes that OBT number is a bit more or less than the calculated barrel time for an accurate load, and the explanation there is that the shock wave is moving towards or away from the muzzle at the time.

Is OBT a range or a number ?