OCW method - second rifle test

[Againstthegrains]

I see it like this.

Imagine you are trying to shoot a target on a park swing that is moving.
If you aim for the target at the top of the upward swing when the object is momentarily stationary, and shoot when it reaches the top, then you have a much better chance of hitting than when the target is at the bottom of the arc of the swing when it is moving very fast.

If you get mathematical, and plot the speed of the swing on a graph as it goes back and forth, it will basically look like a sine wave. In other words, it slows as it reaches the top of the swing cycle and speeds up in between.

Now, when you shoot a bullet, the clever people (not me) figured out that it creates a wave through the barrel, and the tip of the barrel moves in a wave pattern, in other words, just like a swing, so if you release a bullet when the swing is up/ momentarily stationary, the bullet is released from the same point each time and the grouping will be good. If the bullet is released when the barrel is moving rapidly between stationary phases, the chances of the bullet being release from the same point each time is low, and hence the bullets are scattered around, and the grouping is poor.

Now, if you make the ropes of a swing longer, the swing will spend more time being almost still at the top of each passing, and take longer to get to the top of each side. It will also move at a higher velocity at the bottom/fast part of the swing cycle. In other words, changing the barrel length, is like changing the length of the ropes of a swing, and must be done for the OBT to work.

Now if you put it all together, it was a challenge for the developers of the theory to explain where the barrel will be moving fast and when it will be stationary, so they describe it (with whole lot of calculations I don't have a clue about) in terms of the time the bullet spends in the barrel before it is released, knowing that these times will correlate to points along the wave where the barrel is moving fast or is stationary. They called these "stationary" times where the barrel is hardly moving when the bullet is released, accuracy nodes, or Optimal Barrel Time (OBT). These barrel times equate to muzzle velocity or bullet speed. On a normal hunting rifle, the nodes are about 200ft/s apart.

So, if you have Gordon's Reloading Tool or Quick Load, and you can find the node # that fits within the pressure limits for that chambering that gives you a good fill % and powder burn rate with a specific bullet. Once you are there, load up a few tests either side of the predicted velocity. Go to the range, and see which test gives you that measured velocity (in the real world) and there you have it. The bullet should be releasing at the velocity where the barrel is predicted to be almost stationary, and therefor produce the best groups.

The other way of doing it, is the old fashioned way, where you shoot a ladder within the min and max velocities of the reloading guide, find your most accurate load, feed it into Quick Load, and suprise suprise, most of the time, your best grouping occurs at a velocity that equates with an OBT node.

[janfred]

No.

Chris Long specifically developed the OBT theory as it relates to the annular wave cased by the powder explosion. The aim is to get the velocity SD down to a minimum by predicting when the annular wave is clear of the muzzle.

This is distinctly different from the Optimal Charge Weight theory that deals with the harmonic length of the barrel and how it whips around its axis.

[oafpatroll]

Thanks @Againstthegrains, that's helpful. I got the wave form thing but hadn't clocked to the 'momentarily stationary at peak and trough' bit.

[Treeman]

Thanks @Againstthegrains, that's helpful. I got the wave form thing but hadn't clocked to the 'momentarily stationary at peak and trough' bit.

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Ya, i only caught on to that recently as well, like a piston in a motor - makes sense.

[Againstthegrains]

No.

Chris Long specifically developed the OBT theory as it relates to the annular wave cased by the powder explosion. The aim is to get the velocity SD down to a minimum by predicting when the annular wave is clear of the muzzle.

This is distinctly different from the Optimal Charge Weight theory that deals with the harmonic length of the barrel and how it whips around its axis.

I completely agree with you. The swing analogy was more for illustrative purposes and understanding. It is much easier for us mortals to imagine a swing with a stationary phase, than a metal bar with a wave like pulse travelling through it.

[Tripodmvr]

My understanding is that OBT does not reflect the movement of the muzzle, but it calculates when the pressure wave is at or near the knox of the rifle. The pressure wave distorts the shape of the crown so as to form an ellipsoid shape. The node calc is then when the crown shape is least deformed and the wave at the knox. That is also why the node can be as wide as 1gr in width - the wave moving away, reflecting and moving forward again creates this.