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Reloading using Optimum Bullet Time (OBT) – often referred to as Exit Time

I searched this website and didn’t find any reference to Optimum Bullet Time tuning to reduce barrel harmonics.
Since I am new to this forum, I figured I should share some of my findings with respect to the benefits of using this approach when reloading.

I stumbled across Chris Long’s original article on the theory for tuning for OBT in 2014, but it had no data with it.

I spent 2 years gathering data and initially discovered that my ‘shooter induced variations’ were masking the results related to OBT. I also had to pay attention to temperature when shooting temperature sensitive powders since the projected velocities were not the same as experienced velocities.

After two years of testing and improving my shooting technique, I finally had my technique improved enough to begin to identify the effect of changing the bullet Exit Times from right on to away from OBT. I found that the difference in group averages between having the OBT that had the shock wave at the chamber (the optimum desired effect) instead of at the muzzle (the worst effect), resulted in a difference of about 0.090 inches in group sizes.

With one of my best shooting target rifles, a Savage 12 FV with a 26.0 -inch 1:9 twist 0.3% carbon steel barrel factory barrel using a F-Class Sinclair bipod and a Protector rear rest, I tried groups at the 10th reflection (OBT of 1.134 msec.) and the 12th reflection (OBT of 1.361 msec.) of the shock wave at the corresponding OBTs for each reflection. I also loaded some loads off the desired OBTs.
I shot 1,513 groups with the factory barrel and measured 1,444 groups for exit time with the Oryx Chassis with the following results:

With Oryx Chassis and Leupold 45X (factory 0.3% Carbon 26.0-inch 1:875 twist barrel)
OBT
Average
# Groups
Rank
Delta OBT
% of Refl.
Avg Diff
% AVG Diff
10th Refl
1.134
0.2710
163
2
0.000
0.0%
0.0011
0.4%
Fast 12th
1.328
0.3102
15
3
-0.033
-29.3%
-0.0403
-12.3%
12th refl
1.361
0.2699
1266
1
0.000
0.0%
0.0000
0.0%

To put the results in perspective based on performance by bullet weight, I have listed the results of 1,198 5-round groups by bullet and weight. For the factory barrel, the 73 ELD-M bullets performed the best followed by the 77 SMK bullets, 69 SMK bullets and 73 Berger bullets.

Average
St Dev
# Grps
69 SMK
0.270
0.055
285
3
69 TMK
0.287
0.050
134
6
73 Berger
0.271
0.036
32
4
73 ELD-M
0.262
0.048
99
1
77 SMK
0.267
0.054
403
2
77 TMK
0.284
0.049
245
5
0.273
0.052
1198

Replacing the Factory Barrel:
In April 2025, I began to be concerned that the chamber wear on the factory barrel that caused me to increase the OAL in order to maintain a desire range of jumps was beginning to be so far out of the neck as to make consistent neck tension unreliable. As a result, I replaced the factory barrel with a Shilen Select Match .223 Rem 416R 26.125-inch barrel and measured 113 5-inch groups at three different OBTs. Actually, I didn’t choose the two outlying OBTs to test them, I just loaded early test bullets off the desired OBT. But the following results do show the difference in results when loads stray from the desired OBT.
The reason I didn’t load any rounds for the 10th reflection is that I was predominantly focusing on heavier bullets (69 to 90 grains) and know that bullets weighing 77 grains or more are close to or over Pmax when they are loaded at the 10th reflection. They are also very near minimum loads at the 14th reflection, so the 12th reflection was the OBT that fit all the heavy bullet selections.

Admittedly, there are very few samples that I loaded away from the desired OBT, but my previous testing has proven to me that loading at the OBT produces the smallest group sizes, so I don’t normally create loads for ETs that are off the desired OBT. Even this small sample shows that there is a good reason for doing so, especially when I am trying to determine which bullet weights shoot best.

With Oryx and Leupold 45X (Shilen 416R SS 26.125- inch barrel)
OBT
Average
# Groups
OBT Var.
% of OBT
% of 1 Refl
Avg Diff
% AVG Diff
1.296
0.2470
1
-0.009
-0.69%
-8.27%
0.038
18.4%
1.305
0.2087
108
0.000
0.00%
0.00%
0.0%
1.342
0.2410
4
0.037
2.84%
34.01%
0.032
15.5%

As of the last session with the new barrel, the number of groups measured are now at 137 and the performance by bullet weights at the desired 1.305 msec. OBT are as follows:

Weight
Average
# Grps
St Dev
Rank
69
0.220
11
0.020
3
70
0.231
24
0.028
5
73
0.207
24
0.029
1
77
0.219
34
0.034
2
80
0.221
36
0.045
4
90
0.244
8
0.027
6
All Wgts
0.221
137
0.038

The best performers, the 73 grain Berger BTM and Hornady ELD-M bullets, so far have performed the best, with the ELD-Ms (0.207 for 20 groups) performing slightly better than the Berger BTMs (0.211 for 4 groups). Their combined average is 0.012 smaller than the 2nd place bullet weight.
The 2nd to 4th bullet weight averages average within 0.003 thousandths of each other. Those averages could have easily been impacted by my shooter induced variations.

The 80 grain bullet results are biased by one load of Nosler CCs, an older bullet shape, that averaged 0.252. The Berger, ELD-M 80, and SMK 80 grain bullets, all with much higher BCs, averaged 0.217, which would have put the 80 grain bullets in 2nd place. I would conclude that this barrel performs well with bullets from 69 grains to 80 grains.
The 70 grain bullet results, composed of Berger VLD and Nosler RDF bullets are biased by one load of Nosler RDF bullets that was shot with in very hot barrel. Without that one load, the 70 grain bullets, including one load with the Nosler RDFs averaged 0.226.
The 90 grain bullets, all SMKs, may not be stabilizing at 100 yards or just simply are not preferred by this particular barrel. I will be hunting for other 90 grain bullets to see if they might perform better.

I haven’t tried to determine if the twist on the new barrel is exactly 1:7. (My factory barrel, that was specified as 1:9 twist, actually measured 1:8.75 twist so it stabilized 77 grain bullets and shot them very accurately).
 
The barrel reflection frequency depends upon the type of steel.
The factory barrel was 0.3% carbon steel with a reflection speed of 19,107 fps. For a 26-in barrel, that results in a reflection time along the barrel of 0.11340 msec. yielding 1.134 for 10 reflections or 1.361 for 12 reflections.
The Shilen barrel was 416R stainless steel with a reflection speed of 20,014 fps. For a 26.125 barrel, that results in a reflection time along the barrel of 0.108778 msec. or 1.305 for 12 reflections.

Both barrels have a 11 degree recessed crown at the muzzle.

On a different rifle with a 24 inch 0.3% steel barrel, I have a muzzle brake on it.
Without the muzzle brake that barrel has a 12th reflection time of 1.256 msec. With the vanadium steel muzzle brake (19,969 fps reflection speed), the barrel plus muzzle brake has a 12 reflection time of 1.345 msec. (With a muzzle brake, or suppressor, there are no rifling in the attached device, so the final OBT does not include the last pass through the muzzle brake or suppressor since the bullet is already flying free.)

When choosing reloads that match up for the desired OBT, I use QuickLOAD because it provides the bullet exit time. Interestingly, I was unsure if the QuickLOAD number would be accurate.
The results show that it is accurate enough to provide very good and very repeatable results.
 
The barrel reflection frequency depends upon the type of steel.
The factory barrel was 0.3% carbon steel with a reflection speed of 19,107 fps. For a 26-in barrel, that results in a reflection time along the barrel of 0.11340 msec. yielding 1.134 for 10 reflections or 1.361 for 12 reflections.
The Shilen barrel was 416R stainless steel with a reflection speed of 20,014 fps. For a 26.125 barrel, that results in a reflection time along the barrel of 0.108778 msec. or 1.305 for 12 reflections.

Both barrels have a 11 degree recessed crown at the muzzle.

On a different rifle with a 24 inch 0.3% steel barrel, I have a muzzle brake on it.
Without the muzzle brake that barrel has a 12th reflection time of 1.256 msec. With the vanadium steel muzzle brake (19,969 fps reflection speed), the barrel plus muzzle brake has a 12 reflection time of 1.345 msec. (With a muzzle brake, or suppressor, there are no rifling in the attached device, so the final OBT does not include the last pass through the muzzle brake or suppressor since the bullet is already flying free.)

When choosing reloads that match up for the desired OBT, I use QuickLOAD because it provides the bullet exit time. Interestingly, I was unsure if the QuickLOAD number would be accurate.
The results show that it is accurate enough to provide very good and very repeatable results.
Likewise, I've found QuickLoad's OBT to be good enough to help with repeatable results, though I find there's just a little tuning left to do to get the best result. For me, I've found it to be a good time and component saver. :D
 
I researched this extensively several years ago and found the OBT concept "worked" in that it agreed closely with the point of impact nodes as determined using charge weight ladder testing on the target, and using a procedure presented in Vaughn's book the POI results can be analyzed to determine the muzzle frequency involved. The results show the vibration of the muzzle corresponds to the speed of sound which was Long's premise (but not muzzle diameter). OBT works but due to a different reason.

Details are here: https://forum.accurateshooter.com/threads/barrel-harmonics-associated-with-the-target.4083589/
 
I agree that muzzle diameter has nothing to do with it, nor does caliber. The shock wave travels in the steel not in the open air of the caliber.

The magnitude of the muzzle vibration increases as the shock wave moves away from the chamber as the bullet exits the muzzle. Minimizing the vibration eliminates some of the harmonics and allows bullet POI to be more accurate. The speed of the muzzle vibration is not the issue, minimizing the vibration is the issue.
 
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I agree that muzzle diameter has nothing to do with it, nor does caliber. The shock wave travels in the steel not in the open air of the caliber.

The magnitude of the muzzle vibration increases as the shock wave moves away from the chamber as the bullet exits the muzzle. Minimizing the vibration eliminates some of the harmonics and allows bullet POI to be more accurate. The speed of the muzzle vibration is not the issue, minimizing the vibration is the issue.

Vibration can be your friend, manifested as positive compensation. Which is why higher velocity hits higher, sometimes lower, or ideally at the same point.
 
That might be true if the result remained consistent. I'm not sure there would be a way to determine that.

As long as the POI for the groups I am shooting lands at the same point, I would be satisfied.
The POA could be easily adjusted to a desired position by adjusting the scope.
If the POI changes with each shot, the validity of the OBT concept would be debatable.

So far, my results show that the POI for each shot in a group seems to be clustered more closely when the OBT is the at the desired value, so OBT appears to be a practical means of improving accuracy.
That works for me.
 
Likewise, I've found QuickLoad's OBT to be good enough to help with repeatable results, though I find there's just a little tuning left to do to get the best result. For me, I've found it to be a good time and component saver. :D
Early in my testing with multiple rifles, I used the published barrel length in QuickLOAD and I had to tune a bit to get the best result with some barrels.
Then, by chance, I found that the published barrel length is not always accurate.
I have occasionally found variations as much as 3/8 an inch in barrel length compared to the published length, but that is rare.
The new Shilen barrel is 1/8 inch longer than spec.
The Factory barrel it replaced was right on the published length.
Once the measured barrel length was entered in QuickLOAD for a particular rifle, I found that I didn't need to tune for that barrel at all.
 
Early in my testing with multiple rifles, I used the published barrel length in QuickLOAD and I had to tune a bit to get the best result with some barrels.
Then, by chance, I found that the published barrel length is not always accurate.
I have occasionally found variations as much as 3/8 an inch in barrel length compared to the published length, but that is rare.
The new Shilen barrel is 1/8 inch longer than spec.
The Factory barrel it replaced was right on the published length.
Once the measured barrel length was entered in QuickLOAD for a particular rifle, I found that I didn't need to tune for that barrel at all.
This is something I haven't really thought about. So, I just measured my 26" Krieger barrel to see how accurate it is and it's just 1/16th short of being exactly 26".

I do wonder about certain features on the barrel might affect the calculation, like the muzzle end of Heavy Palma contour threaded for a break and the amount of the barrel that threads into the action or a barrel nut and its size that hold the barrel in place (like some drop in barrels) . . .??? It seem maybe that's were the need for a bit of tuning comes from???
 
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For quite a while I only found Long's writing. It was ok for me, but, his premise was a single 'excitation' and assuming all of the barrel movement was due to that. I liked the accelerometer work that showed the multi-faceted frequencies in the barrel harmonics in the other papers.

I also liked the on paper results and how they pertain to the tuning. I've seen that and used it, just had not seen an instrumented barrel to confirm it.

This also saved me a lot of money. I was about to go out and buy an oscilloscope, strain gauges and accelerometers. :)

As a side issue, is there a way to change the barrel steel parameter (speed of sound) in GRT? The documents I've seen indicate the one in the program is for carbon steel barrels.
 
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Strraightshooter1
I would try the calculation with the different barrel length and I think you will find that it changes the OBT by a small amount but it does change it.
The 416R 26.125-inch barrel has a calculated OBT of 1.3053 msec. for the 12th reflection.
For the published length of 26.0 inches, the calculated OBT is 1.2991 msec. for the 12th reflection.
For a 0.3% carbon steel barrel, the 26.125 barrel has an OBT of 1.3673 msec for the 12th reflection.
For the published length of 26.0 inches the calculated OBT is 1.3608 for the 12 reflection.
The 907 ft per second in reflection speed make a bit of difference in OBT doesn't it.

From my data, that change in OBT of 0.0062 msec. with the 416R barrel or the difference of 0.0065 msec in the 0.3% carbon steel is just about at the point of being visible to me, because my 'shooter induced variation' would mask the difference that small.
Your probably younger and steadier and have better sight, so you might be able to see more difference.

I have learned that I am still most of my accuracy problem so I don't get too hung up on super details of adjustment anymore, but I do know that OBT works and I use it every time I reload.

Considering 0.002 change in seating depth or an equivalent change in trim length will make a 0.001 change in OBT, I tend to focus on the stuff I can control and live with the stuff I can't.
 
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charlie b,
First, I want to thank you, and others, for recommending that I look into this forum. I appreciate the advice.

I, too, have an engineering background, but more system design and systems engineering than specific device analysis, but I still tend to over analyze stuff.
But, at some point, I realized that in complex systems there was a point at which there were so many interacting variables that only the major ones could really be controlled, and the rest were in the noise. Most of the remaining minor variables were not going to make a significant difference in performance. The rare exception was when the minor variable was a threat to safety.

In this case, when I found the kind of improvements that good reloading techniques and controlling OBT could produce, I could accept the results and get on with reloading and trying to get smaller groups within my physical limits.
For me, the results that I measured confirmed that I had found the major contributors and had to live with my own personal limits because I wasn't getting any better.
Fortunately, I found that the new barrel reduced some of the limits because it was an outside improvement like those I found in design when technological advancements improved the components available.

With respect to GRT, I don't have the experience with it to comment, but QuickLOAD doesn't provide that feature at all, regardless of the type of steel. But it does provide the value of the exit time, so I can tune for OBT. I have a spreadsheet that I use to calculate the OBT for a new barrel and then use that value as my goal in arriving at reloads when using QuickLOAD. Tuning a reload for OBT tends to require a trade off between powder load, seating depth, trim length, and jump. After a while, I recognized the interactions of those factors and manage achieve my desired OBT pretty easily.
By the way, most of those factors individually have the least impact on accuracy if I keep the OBT on the desired value.
 
In looking at Long's paper, a take away was 'nodes' were +/- 2 usec.
Using GRT, it predicted that in 223 rem it takes about .2 grains of powder to make a 2 usec difference in barrel time => nodes in 223 rem are about +/- .2 grain of powder.

Now that I mention it, it would be interesting to see what seating depth changes would make in barrel time.
 
I don't use GRT, so I can't comment, but that projection doesn't seem to agree with my results with QuickLOAD.
As an example, using a load I just used for my next session with my new barrel for my .223, I find that for a .223 load (with Varget and a 73 gr Berger LTB bullet), a 0.1 grain change in powder load makes a 0.008 msec. change in exit time at the muzzle.
A 0.002 change in OAL or in cartridge trim length results in a 0.001 change in exit time.

For a temperature sensitive powder like H4895 (with a 155 fps change over 1 to 125 deg. F.) a change of 2 degrees F. makes a change of 0.002 msec. in exit time.
For VV N140 (with a 100 fps change over 0 to 125 deg F.), a change of 2 degrees F. makes a change of 0.001 msec. in exit time.
I generally opt to use temperature insensitive powders like Varget that change 4-8 fps over the range of 0 to 125 deg. F., to eliminate having to predict the temperature that I will experience at the range. Even at a change of 10 degrees, the worst temperature insensitive powders only change 0.6 fps so the exit time remains the same.

Given the impact of powder load dominates, I try to find the powder charge that gets me in the range of OBT and then tune seating depth to get the OBT just right, given the trim length of the brass I am using.
I find that staying in a range of 0.010 in OAL doesn't change the jump enough to see a significant difference, and I rarely have to change OAL that much.

I also find that a 0.002 change in bullet length makes a 0.001 msec. change in exit time.
I now measure bullet lengths for each new lot of bullets and change the value in QuickLOAD when I find a difference, after I found more than 0.005 variation in bullet length (base to tip) more often than I would like to admit when changing bullet lots.
 
Like most things in an engineer's/scientist's life, there is only probability we know something.

It is easy for folks to get used to assuming the modulus or density is one solid number for example. It makes teaching math or science cleaner at first.

When we are starting out they hand us simple formulas like M=(Y/p)^0.5 and we are off to the races calculating Mach numbers.

Then we get into the real world and find out that there is a variation in the speed of sound in a material like steel, and that material properties like the Youngs Modulus and the density both have a tolerance range of their own.

For example, the density of 4140 steel depends on where I buy it. It might be 7700 kg/m3 or it might be 8030 kg/m3. The modulus might be 190 GPa or it might be 210 GPa. So it follows that the Mach number ends up having an uncertainty till we get to the lab.

All of which is not to say anything negative against calculating OBT numbers. But what I am trying to say is that in industrial settings, we are not surprised when they have a tolerance that means we wait to measure them in place and then start manipulating the recipe.

The OBT wave is just one of many, and each one has a "Q" based on damping. If the Q for a given mode is low, or sits very close in phase to other ones, it may or may not matter as much. So not every OBT wave value will have an equal effect on your group due to several factors, including the tolerances of each term that go into calculating the Mach number of the steel.

We do our best to design with the math, but unless we know the actual frequencies, along with their phase and damping in relation to the others, we have to be prepared to move the recipe up or down a little to see when the gun agrees to cooperate. YMMV
 
CFJ, GRT has a module for calculating the OBT, and, will calculate a load to get that result (or close to it) as well as modify the powder parameters based on your observed muzzle velocities.

It does get really close, but, still have to work out the final bits with a few ladders.

I am playing with the numbers for my .223 just to see how much better I can get. Problem is I am still struggling with my shooting. Hard to see a difference of 0.1" or 0.2" if you can't hold the rifle on point that accurately :)

I like this kind of stuff. Reminds me of my testing in explosive welding of dissimilar metals. Back when instrumentation was archaic compared to what we have now. Heck my masters thesis back in the 80's was the development of a remote temperature sensor that worked at higher temps. Something you can buy cheap off Amazon.
 
In looking at Long's paper, a take away was 'nodes' were +/- 2 usec.
Using GRT, it predicted that in 223 rem it takes about .2 grains of powder to make a 2 usec difference in barrel time => nodes in 223 rem are about +/- .2 grain of powder.

Now that I mention it, it would be interesting to see what seating depth changes would make in barrel time.
I have been doing just that. Turns out you can get quite a bit of change in barrel time (and vel) with a relatively small change in seat depth. It is iterative, change seat depth, vel changes due to difference in case vol, barrel time changes due to bullet position and vel change. GRT makes it a bit simpler, but, it is not straightforward and I wonder if the change in vel will 'upset the balance' of what the barrel 'likes' in terms of velocity.
 

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