Do you feel that OBT loading is worthwhile?
- Thread starter fatelvis
- Start date
Murray Brook
Silver $$ Contributor
I generally use the OBT when working up a new load. It gets me close most times, but rarely is it right on. It does save me some components in the long run, but your input into the program must be spot on.
charlie b
Gold $$ Contributor
I found that using GRT (along with manuals) shows me where max pressure is at for the bullet and seating depth I am using. I then start 10-15% low and do a ladder to find the two nodes I am interested in. I've found the GRT calculated OBT is sometimes way off, so I basically stopped using it.
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kelbro
Silver $$ Contributor
Same here. The OBT calculated with GRT has been very close to a node on several occasions. Predicted high pressure within a couple of hundredths several times. I was quite surprised at how well it was able to model the loads.
And same here, been using GRT for a few years now. Essential to input actual velocity data for the OBT calculation to predict the node/speed/pressure more accurately. Found it spot on for Berger 185 Juggernauts and 200.20x's. With 155's GRT seems to prefer ~ 2950-2970 fps, the target tells me 3020 fps for my rifle.
kelbro
Silver $$ Contributor
That part was really interesting. GRT made an iniital prediction which was pretty close. After I shot the workup, I then input the measured velocities back into GRT and the new OBT that was calculated was within .1gr of my best group.
Another thing, inputting actual velocities and running the OBT tab really helps if you are using small rifle primers. In my experience (.308), GRT predictions without actual data will generally reflect velocities from large primers as a 'base case' i.e. it will predict higher velocities compared to the same load with small primers.
kelbro
Silver $$ Contributor
I have also found that my case capacity (H2O) varies between my Krieger .223 barrel and my McGowen .223 barrel by ~.3gr. with the McGowen having the larger chamber. Not sure how much that affects the real-life internal ballistics but it does alter the modeling a little.
CF Junkie
Gold $$ Contributor
Absolutely!
I have been loading for OBT since 2014 and have kept extensive data on all the rifles I have loaded for.
There are a few things that you need to be aware of. As a indication of what extensive means, I have loaded over 78,000 rounds since 2,008 and have measured 14,000 5-round groups and recorded all of them.
Things to remember about OBT:
First, temperature can change powder velocities and change OBT so you need to be cognizant of that.
Second, OBT changes with the type of barrel steel.
Many barrels are 0.3% carbon - reflection speed is 19,107 fps.
Pre WWII barrels are often slower 18,916 fps.
Vanadium steel barrels (1040 and 1050 steel) often called chrome steel are 19,969 fps.
Stainless Steel barrels are 20,000 fps.
416R SS barrels are 20,014 fps.
Third, the 'published' barrel length will change the actual OBT. I have barrels that are up to 3/8 inches longer or shorter than the published length. 16 of 42 barrels I calculated for are not at published lengths. You have to measure to find your actual OBT.
Fourth, changes in Seating depth of 0.002 with identical powder loads will change OBT by 0.001 msec. Changes in trim length or 0.002 with identical powder loads will change OBT by 0.001 msec.
Fifth, the OBT occurs when the shock wave is at the chamber (an even number of reflections) rather than at the muzzle (an odd number of reflections) to minimize the harmonics and keep the crown tight when the bullet is leaving the muzzle. Normally, since the reflection velocity is much faster than the speed of the bullet as it accelerates down the barrel, the OBT can be between 10 to 14 reflections. I generally find that for light bullets the 10th reflection might work but for heavy bullets the 10th or sometimes the 12th reflection is over or very close to PMax pressures.
I have found that there is no statistical change in accuracy for Exit Time changes versus OBT that are around +/- 0.005 msec. that I can determine accurately. (My shooter induced variations probably cause that).
It sounds complicated, but I have found I can get a new barrel close to node by calculating and loading for exit time at OBT in the first session. I generally, load 5 rounds on OBT, and 5 rounds + or - 0.005 and 0.010 msec. off OBT, each, to see if the calculated setting is right on. (I don't assume my measurements account for all the variations that might be related to a specific barrel.)
I find that there is about 0.090 inches change in group size at 100 yards between the Exit time at the chamber and the muzzle.
It took me over a year of improving my consistency in shooting after I accounted for minimizing the 5 items listed above to be able to actually see and measure the effects.
The last 'aha' was finding that temperature changes were messing up my calculation of exit times because the powder was either burning slower in cold or faster in the heat. Fortunately, temperature was one of the things I measured and recorded, so I redid my calculations for hundreds of sessions and all the data was adjusted and became clear.
I have been loading for OBT since 2014 and have kept extensive data on all the rifles I have loaded for.
There are a few things that you need to be aware of. As a indication of what extensive means, I have loaded over 78,000 rounds since 2,008 and have measured 14,000 5-round groups and recorded all of them.
Things to remember about OBT:
First, temperature can change powder velocities and change OBT so you need to be cognizant of that.
Second, OBT changes with the type of barrel steel.
Many barrels are 0.3% carbon - reflection speed is 19,107 fps.
Pre WWII barrels are often slower 18,916 fps.
Vanadium steel barrels (1040 and 1050 steel) often called chrome steel are 19,969 fps.
Stainless Steel barrels are 20,000 fps.
416R SS barrels are 20,014 fps.
Third, the 'published' barrel length will change the actual OBT. I have barrels that are up to 3/8 inches longer or shorter than the published length. 16 of 42 barrels I calculated for are not at published lengths. You have to measure to find your actual OBT.
Fourth, changes in Seating depth of 0.002 with identical powder loads will change OBT by 0.001 msec. Changes in trim length or 0.002 with identical powder loads will change OBT by 0.001 msec.
Fifth, the OBT occurs when the shock wave is at the chamber (an even number of reflections) rather than at the muzzle (an odd number of reflections) to minimize the harmonics and keep the crown tight when the bullet is leaving the muzzle. Normally, since the reflection velocity is much faster than the speed of the bullet as it accelerates down the barrel, the OBT can be between 10 to 14 reflections. I generally find that for light bullets the 10th reflection might work but for heavy bullets the 10th or sometimes the 12th reflection is over or very close to PMax pressures.
I have found that there is no statistical change in accuracy for Exit Time changes versus OBT that are around +/- 0.005 msec. that I can determine accurately. (My shooter induced variations probably cause that).
It sounds complicated, but I have found I can get a new barrel close to node by calculating and loading for exit time at OBT in the first session. I generally, load 5 rounds on OBT, and 5 rounds + or - 0.005 and 0.010 msec. off OBT, each, to see if the calculated setting is right on. (I don't assume my measurements account for all the variations that might be related to a specific barrel.)
I find that there is about 0.090 inches change in group size at 100 yards between the Exit time at the chamber and the muzzle.
It took me over a year of improving my consistency in shooting after I accounted for minimizing the 5 items listed above to be able to actually see and measure the effects.
The last 'aha' was finding that temperature changes were messing up my calculation of exit times because the powder was either burning slower in cold or faster in the heat. Fortunately, temperature was one of the things I measured and recorded, so I redid my calculations for hundreds of sessions and all the data was adjusted and became clear.
jelenko
Gold $$ Contributor
Interesting. From Long's paper, it looked like the muzzle was 'stable' for +/- .002 msec.
Very interesting. Thanks
CF Junkie
Gold $$ Contributor
jenenko,
If I was shooting from a rigid fixture instead of using the 'nut behind the trigger' like me, I might have been able to measure a smaller difference. Even then, I would estimate that if you can't consistently average under 0.4 inch groups, you wouldn't see much of a difference. My 'shooter induced variations' mask the subtle changes.
With my .223 12 FV Savage with a Shilen Select Match barrel and a MDT LSS chassis, I have been averaging under 0.220 for over 500 5-round groups and 30% of my groups are under 0.200. The standard deviation in group sizes has to be under 0.040 to rely on measurements unless you have hundreds of data points. With those kind of group averages, group sizes are consistent enough to see the impacts of bullet type, powder type and OBT reflection changes. Even then, it is always a pulled shot or a shooter error that messes up a group and could mess up a conclusion.
If I was shooting from a rigid fixture instead of using the 'nut behind the trigger' like me, I might have been able to measure a smaller difference. Even then, I would estimate that if you can't consistently average under 0.4 inch groups, you wouldn't see much of a difference. My 'shooter induced variations' mask the subtle changes.
With my .223 12 FV Savage with a Shilen Select Match barrel and a MDT LSS chassis, I have been averaging under 0.220 for over 500 5-round groups and 30% of my groups are under 0.200. The standard deviation in group sizes has to be under 0.040 to rely on measurements unless you have hundreds of data points. With those kind of group averages, group sizes are consistent enough to see the impacts of bullet type, powder type and OBT reflection changes. Even then, it is always a pulled shot or a shooter error that messes up a group and could mess up a conclusion.
Being of the age where the desktop computer fit in my shirt pocket and was called a slide ruler I'm confused by the question. I've seen identical rifles shoot vastly different loads. I read alot and base my first test loads based on case capacity comparisons, bore diameter, barrel length and desired target. TESTING?
Accuracy wise? What defines accuracy? I have two AR's, one in 5.56 and one in 6MM ARC, they are both configured as scoped SPR/DMR rifles (special purpose and designated marksman rifles). If they shoot 4" at 300 yards I keep them, if they shoot 5" I figure on getting my butt shot off.
If my 223 bolt rifle varmint shoot more than 1 1/4" at 300 yards I'm back to the drawing board.
Read more, test, observe and modify your loads, no computer required.
CharlieNC
Gold $$ Contributor
OBT is as good as your input information. The nodes occur at approx 3% charge weight intervals and if you go across say 10% you can infact see several nodes in terms of stable point of impact, as with an OCW test. The shock wave travels at the speed of sound longitudinally along the barrel which causes a corresponding vertical/horizontal movement at the muzzle which is manifested in the poi; this effect is clear, I don't know about the hypothesized muzzle diameter change.
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