As you go up in your charge and get closer to "max" pressure your burn rate of your powder changes, as soon as you get close to max quick load pressure QL has a hard time with the powders characteristics and predicting whats going to happen because of whats happening with the powder when it burns as pressure goes up, i have tried to work out how to predict this but it is easier to just shoot the load over a crony. Quick Loads is really a fantastic tool hat I use daily but there are obviously other ways to get to you end goal as well , in the beginning I double checked my QL loads with a ladder but these days I pick my components, load up 3 different charges, shoot them over a corny to get the speed, feed that into QL adjust seating depth a bit and generally thats good enough to hit steel at a mile.
Question on OBT
- Thread starter MrBottleneck
- Start date
As you go up in your charge and get closer to "max" pressure your burn rate of your powder changes, as soon as you get close to max quick load pressure QL has a hard time with the powders characteristics and predicting whats going to happen because of whats happening with the powder when it burns as pressure goes up, i have tried to work out how to predict this but it is easier to just shoot the load over a crony. Quick Loads is really a fantastic tool hat I use daily but there are obviously other ways to get to you end goal as well , in the beginning I double checked my QL loads with a ladder but these days I pick my components, load up 3 different charges, shoot them over a corny to get the speed, feed that into QL adjust seating depth a bit and generally thats good enough to hit steel at a mile.
Joe R
Gold $$ Contributor
When I first start developing a load I revisit all the basic variables parameter (case length, bullet length, temperature, etc...). I usually find that I've changed something in my brass prep and forgot to update QL. Then I test a few charges and get velocities that are about 2 grains apart. I go back home and massage the numbers in QL until it accurately reflect the actual velocity of both charges. If you can't do that, any QL projection are going to be off. So I would recommend revisiting your individual parameter.
As I understand it, please correct me if I'm wrong, a ladder test is an empirical (guided by practical experience and not theory) approach that works hand in glove with OBT. Before chronographs and QuickLoad that was the only way to get to OBT. Now we have the theory that validates the ladder test. It seems to me that its like when the astronomers see a planet/star out there and then the physicists develop the models/formulas that explain the movement/trajectory of that body. If the two don't jive it means that either the formula is wrong or that there are variables not being accounted for. Chris explicitly mentions the variables unaccounted for in the manual. OBT is an approximation and will not be precise, but gets close enough.
Kindest regards,
Joe
The sound theory of a Ladder Test is that your barrel will deflect and move on the firing of the cartridge. To get a self compensating effect that minimizes the impact of varying velocity, you want the bullet to exit when the barrel is moving in the upward direction. Fast bullets exit sooner and with the barrel muzzle pointed lower and tend to offset the extra velocity effect on POI. Slow bullets exit later with the barrel pointed higher, and compensate for the lower velocity.
You identify this barrel moving up condition by shooting a number of rounds at ideally 300 yards while measuring the velocity of each one, and the vertical point of impact of each one. When you plot vertical vs velocity a sweet spot is identified when the POI levels out with increasing velocity, or sometimes even dips a bit. The objective of a Ladder Test is to identify a velocity that is tolerant to variation from shot to shot. With an accurate rifle this flattening out effect is quite obvious.
I am not sure what OBT and OCW is really based on, other than trial and error. That works too of course.
Good posts Joe and Ron.
As in previous posts I've mentioned that I've somewhat abandoned the ladder test and have studied Chris long's theory on OBT and therefore OCW to get an accurate tolerant load.
This , with the aid of QL (a study in itself) and an accurate chronograph (LabRadar accurate to 0.1%,so claimed) getting to the sweet spot is less arduous.
My experience is that the wider variance in velocity the larger the group will be vertically, therefore strive for a low ES.
I've breezed through some of Varmint Al's blog, but the point I do not get or agree upon , is the barrel moving up and down in the vertical axis with the idea of the slow bullets leaving when the barrel is up and the faster ones leaving when the barrel is low, can't get my head around that one. But ladder test reveals that the 'sweet spot' is tolerant of this. I suppose an optical chronograph was used to record velocities, depending on the spacing of the optical cells the error is in the neighborhood of 1%, which at Dasher speeds is ~30 ft/s. With this the theory is compromised by unreliable data.
Further, the barrel oscillates in the x and y axis at varying degrees. This oscillation is akin to throwing a rock into a still body of water , energy waves radiate from the center outwards and along the length of the barrel.
Now , when the cartridge is fired, a radial shock wave is produced that travels to the end of the barrel and back to its source, this is between 19 and 20K ft/s , depending on the steel. This shock wave , if you can envision an egg going through a snake, varies the diameter of the barrel as the energy of the wave passes any given point in the barrel.
So, the idea is to have the bullet exit at the point of least deformation of the muzzle, this would be the the tightest spot, there is only one 'tightest spot'.
If the barrel 'whips' around like a fishing rod then you may have some mechanical issues with the rifle, such as less than square barrel to receiver mating among others.
Maybe that is why barrels are free floated as there is no obstruction/dampening force to alter the natural frequency of the barrel, this may give the consistency required for tight groups.
It makes perfect sense to me, and the targets sure indicate that is what is happening.
The problem I have with the Chris Long theories is that a shock wave racing down the barrel and distorting the diameter of the bore at the muzzle, is easy to visualize, and probably does happen, but I would suggest the magnitudes of the energy and the actual changes in muzzle bore are minuscule. They are most likely a curiosity but not a significant effect on accuracy. They certainly do not explain the self compensating effect on varying velocity in the sweet spot.
Pardon both my ignorance and inexperience, but I wonder if the military did the type of studies you all are talking about, shock wave going down the barrel and distorting it, for the guns on battleships and ground artillery. Seems to me that if it is a sound theory, it certainly would apply to bigger calibers as well. Maybe the accuracy of one over the other makes the question laughable, but maybe not so much. Just asking?
D
From page 7 of the QuickLoad manual, in fact the "Forward", first paragraph "The first propellant specification were derived from four military small to medium caliber powder types, gun data were taken from anti-aircraft gun barrels...."
Joe R
Gold $$ Contributor
BCBRAD, I like the way you think. I believe most people buy into this because of slow motion videos showing the barrel going up and down. However, I believe the motion may also have a side to side aspect that is not visible. I'm theorizing here, but I believe that the whipping is caused by the stress/pressure created by bullet engaging and traveling the rifling to force it to spin. However, the whipping is not just up and down but more conical. If videos were shot from above the barrel I believe that the side to side movement would be visible.
Brad, I really like your analogy of the egg going through a snake and I am in total agreement with it. IMO the key to precision is all about, as you so well put it "have the bullet exit at the point of least deformation of the muzzle, this would be the tightest spot. I disagree that there is only one 'tightest spot. I'm thinking that there may be more than one tightest spot, however those pseudo nodes are probably not as wide as a true node. The vibrations travel through the steel much faster than the bullet, but are also interacting with the egg and thereby creating other tight spots.
IMO the whipping of the barrel does not take place until after the bullet has left the crown of the barrel and the stress/pressure created by it is released in the opposite direction of the spin. The barrel does whip, search the YouTube videos for "barrel whip" and you'll see it. But not until the bullet has left the scene.
Kindest regards,
Joe
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Ned Ludd
Silver $$ Contributor
Don't need a camera. Just need to work up a load on an OBT node and see how it shoots. After purchasing Quickload, I found [retroactively] that several extremely good loads I had worked up using another method [ladder] were spot on OBT nodes. These were loads in two different calibers, with different bullets, and different powders. No possible way that was simply chance. From that point on, I have used QL predictions and OBT nodes whenever possible. Don't particularly care whether Chris Long's explanation of the mechanism is correct or not...it works.
The process is simple: using as many setup/load-specific inputs as possible (i.e. barrel length, bullet OAL, case volume, COAL, case trim length, etc.), set up a QL file for your bullet/powder/cartridge. Adjust charge weight to find MAX pressure for your cartridge, then back off until barrel time matches an OBT node for your barrel length that is under MAX pressure. Drop the charge weight by about 2%, load some rounds, and determine an average velocity. Using the program, adjust the burn rate (Ba) until predicted and actual velocities match exactly. Now you have "calibrated" QL to your setup for the specific lots of powder/primers, barrel length, brass, and bullets your are using. Now you adjust charge weight to find the closest barrel time to the OBT node possible. When adjusting charge weight in 0.1 gr increments, the program does not usually hit the optimized barrel time exactly in all decimal places, but one charge weight will be closer than any other. I usually try to pick the one just a tick slower. For example, OBT node 4 for a 30" barrel from Chris' Table is 1.3684 ms. I have worked up a number of very good .308 loads that generally have fallen from about 1.368 - 1.375 ms; very close but ever so slightly slower.
Once you have calibrated QL and identified the predicted OBT charge weight, work up a test series in 0.1 gr increments with the predicted OBT charge weight centered. I usually try to test two to three 0.1 gr increments to either side of center. As a readout, I primarily use velocity data (i.e. looking for the low ES/SD window, which seems to correlate very well with the OBT node). However, I also pay attention to grouping at 100 yd, which also seems to correlate very well with the OBT node. Even if the groups aren't the greatest, you will fix that in the next step via optimization of seating depth. But it is key to identify the minimal ES/SD window in the charge weight testing phase. In my hands, the actual optimized barrel time charge weights almost always fall within 0.1 - 0.2 gr of the predicted value.
This process has worked very well for me in terms of developing load for my F-TR rifles. I routinely end up with loads capable of 5-shot groups at 100 yd in the 1/4 - 1/3 MOA range that can maintain their velocity/vertical POI during the course of 20+ shot strings of fire. I do not shoot BR, although I know their loading approaches are somewhat different than most F-Class shooters, and I have seen a number of posts indicating marked skepticism regarding the use of OBT loading such as I have outlined here for BR shooting. The precision requirements and match shooting formats used are different, so it may not work well for BR. However, it works very well for F-Class, so the choice to try it must be left up to the individual.
The main advantages of this approach are: 1) you have a defined goal (optimized barrel time node) for which QL generates an output (barrel time) strictly based on barrel length; and 2) it can save a fair amount of time, effort, and components by allowing you to go to fine increment charge weight testing with only a minimal up front effort (initial calibration step). It is not magic or any kind of miraculous load development process. You still have to load rounds, determine velocities, and assess groups on the target. However, it can save some time/effort and it does give you a defined target to achieve in terms of barrel time; as opposed to more classic approaches where you start out with coarse charge weight testing because you may not have a very good idea initially where the load will likely end up. So you shoot a wide charge weight window to find out, then go back and test across the relevant [narrower] range in smaller increments. OBT nodes and QL can generally save you some time on that first step. After that, most methods are pretty much the same.
The process is simple: using as many setup/load-specific inputs as possible (i.e. barrel length, bullet OAL, case volume, COAL, case trim length, etc.), set up a QL file for your bullet/powder/cartridge. Adjust charge weight to find MAX pressure for your cartridge, then back off until barrel time matches an OBT node for your barrel length that is under MAX pressure. Drop the charge weight by about 2%, load some rounds, and determine an average velocity. Using the program, adjust the burn rate (Ba) until predicted and actual velocities match exactly. Now you have "calibrated" QL to your setup for the specific lots of powder/primers, barrel length, brass, and bullets your are using. Now you adjust charge weight to find the closest barrel time to the OBT node possible. When adjusting charge weight in 0.1 gr increments, the program does not usually hit the optimized barrel time exactly in all decimal places, but one charge weight will be closer than any other. I usually try to pick the one just a tick slower. For example, OBT node 4 for a 30" barrel from Chris' Table is 1.3684 ms. I have worked up a number of very good .308 loads that generally have fallen from about 1.368 - 1.375 ms; very close but ever so slightly slower.
Once you have calibrated QL and identified the predicted OBT charge weight, work up a test series in 0.1 gr increments with the predicted OBT charge weight centered. I usually try to test two to three 0.1 gr increments to either side of center. As a readout, I primarily use velocity data (i.e. looking for the low ES/SD window, which seems to correlate very well with the OBT node). However, I also pay attention to grouping at 100 yd, which also seems to correlate very well with the OBT node. Even if the groups aren't the greatest, you will fix that in the next step via optimization of seating depth. But it is key to identify the minimal ES/SD window in the charge weight testing phase. In my hands, the actual optimized barrel time charge weights almost always fall within 0.1 - 0.2 gr of the predicted value.
This process has worked very well for me in terms of developing load for my F-TR rifles. I routinely end up with loads capable of 5-shot groups at 100 yd in the 1/4 - 1/3 MOA range that can maintain their velocity/vertical POI during the course of 20+ shot strings of fire. I do not shoot BR, although I know their loading approaches are somewhat different than most F-Class shooters, and I have seen a number of posts indicating marked skepticism regarding the use of OBT loading such as I have outlined here for BR shooting. The precision requirements and match shooting formats used are different, so it may not work well for BR. However, it works very well for F-Class, so the choice to try it must be left up to the individual.
The main advantages of this approach are: 1) you have a defined goal (optimized barrel time node) for which QL generates an output (barrel time) strictly based on barrel length; and 2) it can save a fair amount of time, effort, and components by allowing you to go to fine increment charge weight testing with only a minimal up front effort (initial calibration step). It is not magic or any kind of miraculous load development process. You still have to load rounds, determine velocities, and assess groups on the target. However, it can save some time/effort and it does give you a defined target to achieve in terms of barrel time; as opposed to more classic approaches where you start out with coarse charge weight testing because you may not have a very good idea initially where the load will likely end up. So you shoot a wide charge weight window to find out, then go back and test across the relevant [narrower] range in smaller increments. OBT nodes and QL can generally save you some time on that first step. After that, most methods are pretty much the same.
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Varmint Al has done the most credible work on this subject. His finite element analysis models show that the gun is forced to move by the firing, and while the barrel in a center fire gun does not have time to establish a fundamental vibration frequency, the barrel does move before the bullet exits. On rimfires the barrel time is about three times longer, and there is more time to establish a vibration mode.
Are you saying the barrel only whips in the 'y' axis?
It is my personal theory, that the y-axis is the predominant mode of movement. Remember that a true ladder test is only intended to minimize the effect of velocity variation on POI vertical. Velocity variation is essentially vertical movement. You do the ladder test first to get velocity out of the way. Then you focus on minimizing ES and SD so you stay within the velocity band. And of course you optimize jump/jam. For those options it is better to focus on group size than on just the vertical. I use 3 groups of three to evaluate jump/jam. I've seen it stated that you can use to the ladder test to optimize jump and jam. I do not believe that is correct.
Now if you do the ladder test, and optimize your velocity, and all your groups were horizontal strings, then I might start to believe the y-axis vibration is not the predominant mode. I have not seen that. When you minimize the vertical you minimize the group in my experience.
Very good explanation!!!
Savagedasher,
If you pick a powder that the Z1 and Pmax Are one Line on or before the pressure the pressure curve it will always shoot . Larry
Will you please give us an example of this?
Thank you.
Harry
If you pick a powder that the Z1 and Pmax Are one Line on or before the pressure the pressure curve it will always shoot . Larry
Will you please give us an example of this?
Thank you.
Harry
I have mentioned this before. Not new, but correct.
I got a book written by a fellow named Mincham in Australia. He outlines, in my opinion, the best way to take advantage of OBT. His method carries over to "adjusting" the length of the barrel, but I use a tuner for that.
I start by "characterizing" the barrel. I load five rounds each of 5-6 different loads from low to near maximum. I also use a jump consistent to what I have used before successfully. I take off the tuner (if you use one) and set-up a chronograph. I then shoot at 5 targets at typically 200 yd in round robin fashion. I take my time at this so the barrel doesn't over-heat. I use 200 yd as it widens out the group a bit making it easier to measure as well as still being not to difficult to shoot good groups.
I record each shot Mv and analyze each target with OnTarget. I calculate the Average Mv, SD and ES of each five shot group. I then use EXCEL to plot the ES vs loads and additionally do a curve fit on the curve drawn by Excel of 3 rd-5 th order polynomial. This curve fit does a good job of finding critical information between load points. The resultant curve finds the OBT exactly as there will be one or more minimums of ES. The low ones will probably surprise you as they can be VERY low! This is then the load I use to tune my tuner with. If you don't use a tuner, check the groups each load shot. Ideally, the best group matches the lowest ES, but maybe not. But chances are, the best group is good. At this point, the author of the book outlines how to calculate how much to shorten your barrel to take advantage of your data.
I have also tried to fine tune the jump after doing this test and found I could not improve on the load developed above. The performance degraded slightly as I moved away from the test jump. I believe adjusting the jump is simply a fine tune on a load not perfectly chosen in the first place.
I got a book written by a fellow named Mincham in Australia. He outlines, in my opinion, the best way to take advantage of OBT. His method carries over to "adjusting" the length of the barrel, but I use a tuner for that.
I start by "characterizing" the barrel. I load five rounds each of 5-6 different loads from low to near maximum. I also use a jump consistent to what I have used before successfully. I take off the tuner (if you use one) and set-up a chronograph. I then shoot at 5 targets at typically 200 yd in round robin fashion. I take my time at this so the barrel doesn't over-heat. I use 200 yd as it widens out the group a bit making it easier to measure as well as still being not to difficult to shoot good groups.
I record each shot Mv and analyze each target with OnTarget. I calculate the Average Mv, SD and ES of each five shot group. I then use EXCEL to plot the ES vs loads and additionally do a curve fit on the curve drawn by Excel of 3 rd-5 th order polynomial. This curve fit does a good job of finding critical information between load points. The resultant curve finds the OBT exactly as there will be one or more minimums of ES. The low ones will probably surprise you as they can be VERY low! This is then the load I use to tune my tuner with. If you don't use a tuner, check the groups each load shot. Ideally, the best group matches the lowest ES, but maybe not. But chances are, the best group is good. At this point, the author of the book outlines how to calculate how much to shorten your barrel to take advantage of your data.
I have also tried to fine tune the jump after doing this test and found I could not improve on the load developed above. The performance degraded slightly as I moved away from the test jump. I believe adjusting the jump is simply a fine tune on a load not perfectly chosen in the first place.
Savagedasher,
If you pick a powder that the Z1 and Pmax Are one Line on or before the pressure the pressure curve it will always shoot . Larry
Will you please give us an example of this?
Sorry, I'm not SD but this may help. This is the QL graph of the load I use. PMAX and Z1 are one line and at the perfect place in the pressure curve. This load shoots in the .1-.2's when I do my part. I am not a bench shooter. I am a field shooter, so in the hands of a real bench shooter I am sure this rifle / load would be in the zero's. This load was developed using QL only but running the data out of the OBT Tool there is very little difference. It appears there is a fairly wide velocity / accuracy window here for this combo.
Thanks, very much...that's what I thought. I'm shooting a .260 AI and went back over a list of powders. Re-16 will do it with a non-compressed load; IMR 7828 SCC and IMR 4831 do it but the loads would be slightly compressed.
Again, thanks!
Again, thanks!
Check MRP it was the best with 140 GR .
22 was good but it had a bite and didn't like temperature changes . Larry
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