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:
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.
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.
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:
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).
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).