Re: 308...OCW help
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbohardtop</div><div class="ubbcode-body">Tripwire...may be a dumb question but trying to learn...if at 100 yards and a load shows promising on a group even though this is using OCW, wouldn't this load also provide a better group/consistency at longer distances? Thanks for your inputs. </div></div>
It's not necessarily a 100 yards vs. longer thing in relation to some finite degree of "accuracy". 100 yards doesn't prove much to me because the dispersion between charge weights isn't defined enough. I want to more clearly see that difference, and that's more easily done at a longer range, 200 or 300 yards. At these distances a true OCW stands out much clearer. Accuracy nodes will still stay put, movement of groups from barrel occilations are more pronounced.
Regarding also your question, you can hit a good load, on any given day, with any given conditions....but will that load repeat on another day under different conditions, or with slightly different components (another lot of primers/powder/bullets)?
The purpose of doing an OCW workup is to find a point in the load parameters that puts the bullet exit from the muzzle at a point in time when the effects of barrel harmonics have the least effect on the muzzle. IOW, the muzzle occilation is at it's calmest state.
The physics here involves a shock wave, induced by the "explosion" of the powder charge, that literally moves the metal of the barrel. It turns the barrel into a figurative "wet noodle". The reaction the muzzle has is a random occilation that in very small degrees points the muzzle to different places when the shock wave is present. This shock wave slings back and forth from chamber to muzzle several times before the bullet reaches the muzzle and exits. It also, from the moment it starts, begins to dissipate like a tuning fork does.
This is easy enough to see on paper with varying points of impact through a string of consecutive powder charges, then turning to the same point of impact for a grain's worth or so, then going back to varying points of impact. The charges that hit the same POI with the same POA are the "nodes" we look for, and thus the point in time when the shockwave is not emediately upon the muzzle, and the muzzle is moving at a lesser/least amount.
All this is to gain the advantage of getting by the random effects the barrel harmonics have on our bullet but there are also default benefits that are achieved. When we find a "node" we also find a consecutive range of powder weights, which is also by default a consecutive range of pressures, which is also by default a consecutive range of velocities.
If you can apply a range of pressures to a bullet and still achieve the same result per "accuracy", then what you have done is incorporated a degree of tolerance to the load. If all things else are as equal as you can make them per case capacity, per carefully weighed charges, per a consistant lot of components, etc; then this load should perform very well across a fairly wide range of ambient temperatures since temperature is a decisive factor relating to pressure.
By the same token, since we have a tolerance to a range of pressures, such things as case capacity take on a lesser importance because we now have a fairly wide margin of error +/- a half grain or more above and below our load.
Although it's prudent to check a new batch of primers/bullets/powder against an old lot number, these slight changes will also have less meaning than they might otherwise have.
Running OCW has all but eliminated the need for me to sort cases, worry about summer loads vs. winter loads, fret over buying all the same lot numbers, etc. My standard for what I shoot is 1 MOA or less "accuracy", usually it's quite a bit less and in the 1/4 to 1/2 MOA throughout the effective range of the chambering. What this means is, my hunting loads, which are also my play loads, will usually give me the same trajectory in my AO to the degree that I won't have to do much scope adjustment from one time of the year to the next, if any at all. My 85 grain TSX load in my .243AI will do the same on groundhogs in June as it does on deer in November per my dope sheet. True I don't live in Alaska or the Bahamas, but then again that's probably on purpose. I of course shoot enough to see seasonal changes in my loads but that's pretty rare.
Finding a powder charge that gives us the pressure range is the first step. As stated earlier, the groups may or may not be satisfactory...but we know we are in a good node because we know we are exiting the bullet very near to the optimum point in time. Due to the law of averages we have no choice but to pick the exact middle of that pressure range, then fine tune from there.
It's my belief that changing the COAL to fine tune a load has very little to do with pressure that has any practical bearing on the bullet, afterall we just found a whole grain's worth of pressure that the system will tolerate. Moving the bullet in or out one or two or three thousandths doesn't increase or decrease the pressure enough to override that entire grain's worth of pressure.
What it does do though, is change the DISTANCE that the bullet has to travel to get to the muzzle. This relates entirely to time over distance, which WILL affect the period of time that the bullet exits the muzzle in relation to where the shockwave is. If our "groups" in our OCW are slightly sloppy, or not round enough, then by changing the distance the bullet travels by a very small amount, we are adjusting the exit time to allow the shockwave to get back far enough away from the muzzle. This is why I say that COAL adjustments are usually very small, because in the whole scheme of things we are playing with a shockwave that moves back and forth VERY fast. It only takes very small adjustments to refine the optimum window in which the bullet needs to exit with the least amount of affect from harmonics.
