OBT method, barrel length vs velocity

[Stammster]

I have taken some interest in Chris Long's OBT method.

I have several guns in 270 Win, with different barrel lengths. With multiple calibrated Quickload runs, I found the required charge to give the recommended ideal OBT node(s). I did not vary powder, bullet, COAL, H2O capacity, etc.). Just barrel length.

I have come to the conclusion that the OBT nodes are independent of barrel length and a function of muzzle velocity only.

For example, 20", 22", and 24" all have separate ideal recommended optimum barrel times, but they all occur at the same muzzle velocity for a given "node".

Anyone else come to the same conclusion based on your data analysis?

By the way, I am not convinced that the OBT method is actually useful. In fact, I history matched multiple (>15) load developments (0.3 gr powder increments from min to max with 3-5 shot group size) shot over the last few years, and in a more than a few cases, the worst group size occurred at a charge weight/velocity that was supposed to be at the ideal OBT node. In other cases it was pretty close.

 

[DJ Fergus]

You already know more about this than I do but I'm tagging in for more education on the subject.

 

[Tripodmvr7]

I have done more than 1000 calcs for shooters and would be interested in looking at your figures.

If you supply the following I can do a recalculation and vet the data.

1. Measured barrel length in mm as it is the easiest - use cleaning rod and measure from cocked bolt to muzzle tip.
2. COL as shot - QL needs from base to tip of bullet - jump allowed is helpful to see if bullet is jammed in the lands.
3. Bullet used - number on box is helpful.
4. Water case volume measured with fired case that has not been resized in any way - weight in grains is easiest. Weigh empty case and then fill with slightly soapy water till water is level at case mouth. Weigh again and difference is the case volume. Do five to get a mean and idea of variance.
5. Type of propellant and charge weight.
6. Speed obtained and what method - optic chronies I would need distance from muzzle - I advise 5m as muzzle blast sometimes gives false readings.

You have not stored any of the calcs that you have done? If so I can provide an email address and you can send them directly.

Below is an example of a calculated prediction. In practice the burning rate (BRF) at top right is normally higher and needs to be calibrated to actual speed achieved. COL and other relevant figures are defaults as supplied by the program. You can see that the barrel times have been matched as per the OBT theory.

 

[Mikecr]

OBT is not tuning. Lend no credibility to anyone thinking it is.
Instead, it's really about calculated barrel times to AVOID (inverse of that proposed).
Vibrations to avoid being at muzzle on bullet release.

OBT should be referred to as Bad Barrel Timing, BBT, and report only on that.
Someday we'll have a similar calc for best bullet seating. Again, this will not be tuning, but a prerequisite condition better or worse.

 

[Tripodmvr7]

You have a very negative outlook on QL. I have had a 95% record of getting rifles shooting. QL is not an absolute solution but a very good guide line and normally gets you within 1 gr or less of the optimum charge.

Many factors influence the calculations and a bull barrel normally contradicts some findings as the stiffness ensures wider bands of accurate loads. Mikecr, give me examples of some of your loads and lets see if they do not fit the OBT theory. I would need the info as requested above.

 

[asd9055]

You have a very negative outlook on QL. I have had a 95% record of getting rifles shooting. QL is not an absolute solution but a very good guide line and normally gets you within 1 gr or less of the optimum charge.

Many factors influence the calculations and a bull barrel normally contradicts some findings as the stiffness ensures wider bands of accurate loads. Mikecr, give me examples of some of your loads and lets see if they do not fit the OBT theory. I would need the info as requested above.

I am with you. I use QL, then Calibrated QL with capacity, MV and Temperature to get close to a node, then fine tune with adjusting the powder. OBT is real. There is at least one FEA lab that can show it. Don't waste your time arquing with closed minded people. You will never change their mind.

 

[Hawk in WY]

How about we focus on the original observation:

For example, 20", 22", and 24" all have separate ideal recommended optimum barrel times, but they all occur at the same muzzle velocity for a given "node".

i am not at my computer with QuickLOAD but this should be easy to confirm. Unexpected finding for sure. My understanding of OBT would not have lead me to this conclusion. But what would I know?

As to the second topic, I start every load development with QuickLOAD and then cross check with published data. Works for me. For safety, I usually work up to the second lower node until I know case capacity and velocity.

 

[asd9055]

I have taken some interest in Chris Long's OBT method.

I have several guns in 270 Win, with different barrel lengths. With multiple calibrated Quickload runs, I found the required charge to give the recommended ideal OBT node(s). I did not vary powder, bullet, COAL, H2O capacity, etc.). Just barrel length.

I have come to the conclusion that the OBT nodes are independent of barrel length and a function of muzzle velocity only.

For example, 20", 22", and 24" all have separate ideal recommended optimum barrel times, but they all occur at the same muzzle velocity for a given "node".

Anyone else come to the same conclusion based on your data analysis?

By the way, I am not convinced that the OBT method is actually useful. In fact, I history matched multiple (>15) load developments (0.3 gr powder increments from min to max with 3-5 shot group size) shot over the last few years, and in a more than a few cases, the worst group size occurred at a charge weight/velocity that was supposed to be at the ideal OBT node. In other cases it was pretty close.

That makes full sense to me. We all know that the longer the barrel, the higher the velocity with the same bullet and charge. If all nodes happen at same velocity, means you have to use less charge in 22 than 20 and less in 24 than 22 to get the same velocities, that affecting barrel time. Very simple!

 

[Tripodmvr7]

Here is data for a 223 with 53TSX in different barrel lengths.

24 inch Node 3 1.022ms Speed 3240fps Pressure 50130 psi
22 inch Node 3 0.939ms Speed 3250fps Pressure 53980 psi
20 inch Node 3 0.855ms Speed 3263fps Pressure 58760 psi

Powder charges are of course upped to get more pressure and the speed is nearly the same.

If powder charge is kept the same, then

24 inch Node 3 1.022ms 3240fps
22 inch Node between 3 and 4 0.970ms 3178fps
20 inch Node 4 and very close to it 0.917ms (Calced Node 4 = 0.924ms) 3108fps

One can see that, say for instance factory ammo that shoots well in a 24 inch barrel, will also perform in the shorter barrels.

 

[Stammster]

I have done more than 1000 calcs for shooters and would be interested in looking at your figures.

If you supply the following I can do a recalculation and vet the data.

1. Measured barrel length in mm as it is the easiest - use cleaning rod and measure from cocked bolt to muzzle tip.
2. COL as shot - QL needs from base to tip of bullet - jump allowed is helpful to see if bullet is jammed in the lands.
3. Bullet used - number on box is helpful.
4. Water case volume measured with fired case that has not been resized in any way - weight in grains is easiest. Weigh empty case and then fill with slightly soapy water till water is level at case mouth. Weigh again and difference is the case volume. Do five to get a mean and idea of variance.
5. Type of propellant and charge weight.
6. Speed obtained and what method - optic chronies I would need distance from muzzle - I advise 5m as muzzle blast sometimes gives false readings.

You have not stored any of the calcs that you have done? If so I can provide an email address and you can send them directly.

Below is an example of a calculated prediction. In practice the burning rate (BRF) at top right is normally higher and needs to be calibrated to actual speed achieved. COL and other relevant figures are defaults as supplied by the program. You can see that the barrel times have been matched as per the OBT theory.

Thanks for the offer. I'll post up actual data in a few days. I've got everything you asked for already, but scattered around in various notebooks and targets.

In the mean time, this should not be necessary to comment on the observation of barrel length vs identical muzzle velocities vs ideal barrel node time(s).

 

[Stammster]

Here is data for a 223 with 53TSX in different barrel lengths.

24 inch Node 5 1.022ms Speed 3240fps Pressure 50130 psi
22 inch Node 4 0.939ms Speed 3250fps Pressure 53980 psi
20 inch Node 3 0.855ms Speed 3263fps Pressure 58760 psi

Powder charges are of course upped to get more pressure and the speed is nearly the same.

If powder charge is kept the same, then

24 inch Node 5 1.022ms 3240fps
22 inch Node between 5 and 4 0.970ms 3178fps
20 inch Node 4 and very close to it 0.917ms (Calced Node 4 = 0.924ms) 3108fps

One can see that, say for instance factory ammo that shoots well in a 24 inch barrel, will also perform in the shorter barrels.

Those are all node 3 barrel times vs barrel length, aren't they? Are we just using different terminologies? See attached chart.

 

[Tripodmvr7]

Sorry, my slip up, as they are all node 3 - you are correct.

The next set of data should be Nodes 3 and 4.

Have rectified that on my original post.

 

[Stammster]

Sorry, my slip up, as they are all node 3 - you are correct.

The next set of data should be Nodes 3 and 4.

Have rectified that on my original post.

Sorry, my slip up, as they are all node 3 - you are correct.

The next set of data should be Nodes 3 and 4.

Have rectified that on my original post.

Ok, so I think we are thinking on the same page.

Let me tell you where I started, and why the interest. And then where I'm possibly going.

1.). When developing loads for the same gun but with different powders and bullets, I often found accuracy around the same velocity.
2.). When developing loads for several guns of the same caliber, with different barrel lengths, I found accuracy at the same general velocity, albeit charge weight varied. Depending on bullet weight and barrel length, I could often stay in the same node / velocity, but other times I could not.

The above was observed loading 270 Win, but I also load for 30-06 and 25-06. I haven't analyzed any of that data yet.

Wouldn't it be nice if it all boiled down to something like...Aim for these velocities xxxx, yyyy, or zzzz fps during load development. Independent of barrel length, powder, bullet, or possibly even caliber. - Rather than the OBT time / velocity / QL simulation "dance".

Perhaps wishful thinking, as I've only played around nodes # 3-5 and 20-24" barrel lengths - but I suspect that covers 90% of all bolt action hunting sportster rifles.

As mentioned, maybe the factory ammo manufactures already know this.

 

[cape cove]

OBT is not tuning. Lend no credibility to anyone thinking it is.
Instead, it's really about calculated barrel times to AVOID (inverse of that proposed).
Vibrations to avoid being at muzzle on bullet release.

OBT should be referred to as Bad Barrel Timing, BBT, and report only on that.
Someday we'll have a similar calc for best bullet seating. Again, this will not be tuning, but a prerequisite condition better or worse.

Mike
Could you expand on this. I certainly trust your take on this but would like to know how you arrived at this conclusion. In my experience using QL the OBT concept has been hit and miss, but has gotten me close enough in enough instances to take notice.