"Optimal Charge Weight" and the acronym OCW as regards the reloading of metallic firearm cartridges are the intellectual property of Dan Newberry. If using these terms in your own writings, please refer your readers to this webpage for concept clarification. Thanks!
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Dan Newberry's Optimal Charge Weight Load Development...
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What is an Optimal Charge Weight load?
With any given bullet and powder combination, there will be a specific amount of that powder which will cause the bullet to exit the muzzle at the "friendliest" portion of the vibraion cycle. This does not necessarily correspond with the tightest velocity figures, however. Many load recipes have over the years become the "go to" recipes for trying out a new rifle. If your rifle fails to shoot one of these "tried and true" recipes well, you may well have a rifle integrity problem. Consider the Federal Gold Medal Match .308 win ammo. How can one recipe shoot so well in so many rifles? That's what we're after in the OCW load--a load recipe so stable and predictable that it does indeed rise above, and to some extent disprove the "all rifles are different" adage of lore...
If you subscribe to the philosophy that "all rifles are different," and therefore need individual loads tailored to accommodate their idiosyncrasies, how do you explain the near universal MOA performance of loads such as the Federal GMM?
Universally good recipes do exist. Upon realizing this, my questions have been "How do such recipes work?" and "How can I identify such a recipe?"
Theories vary as to why the OCW load works. Chris Long, a friend and engineer has the most plausible idea as to why OCW loads work. He says that the acoustic shock wave which runs end-to-end down the barrel, repeating at around 18,000 feet per second, will disturb accuracy when it is near the muzzle. Therefore the idea is to cause the bullet to exit the muzzle when the shock wave is well away from the muzzle, back toward the chamber. Finding a particular powder charge that will allow this is what OCW reloading is all about.
An example OCW load development follows:
The rifle was my old Remington 788, a 243 Winchester. The bullet was the Sierra 60 grain HP, and the powder used was IMR 3031. (IMR 3031 is an excellent powder for the .243 Winchester with 80 grain and lighter bullets, but you couldn't prove it by the majority of load data manuals--most of them simply overlook it!) Primers were the CCI 200's, and brass was all twice fired Winchester.
In checking the available load data sources, I noted that 39 to 41 grains of IMR 3031 was the suggested max charge zone for IMR 3031 with 60 grain bullets in the .243. I have found that the best loads for all of my own rifles have been loads that were at or near listed maximums, but I do test about 5 to 8 per cent below for pressure signs, then work up.
I began with three rounds each of five different charges, graduating in .3 grain increments. The tested charges were 39.2, 39.5, 39.8, 40.1, and 40.4 grains.
I cleaned the rifle, and shot two fouling shots using IMR 3031 at 39.2 grains, and allowed two minutes for it to cool. I had five targets posted at 100 yards, one target for each charge variation.
I shot one round of the 39.2 grain charge at target 1, allowed two minutes for cooling, and then fired one shot of the 39.5 grain charge at target 2, cooled, one shot of the 39.8 grain charge at target 3, cooled, you get the idea...
This is basically a "round robin" system of grouping each charge, which precludes disadvantaging late groups due to fouling, or heat build up.
When finished, I had three shots on each of the five targets, BUT (!)... I don't look for the tightest group of the five and call that my load.
Here's what I do instead, and I'll explain why later: I look for the three groups of the five that come the closest to hitting the target in the same position. I noted that in this case, the three center groups were the ones which happened to hit the same POI, (all within about 3/4") with the 39.2 grain group hitting low and left from that common POI by an inch. The 40.4 grain charge, in addition to moving high and right of the common POI for the three center charges, opened up in size to about 7/8". The 39.2 grain charge shot about 1/2", but as I said, it wasn't near the common POI that the 39.5, 39.8, and 40.1 charges shared.
So I concluded that the best charge for this application was 39.8 grains of IMR 3031. This was the charge that would allow 39.5 grain charges and 40.1 grain charges to group right with it.
Why would I want that?
Well, variations in brass cases, powder lots, outside temperature--and other things can cause your load to increase or decrease in pressure. With the OCW load, you're covered for a significant amount of pressure differences that may come into play. If I'd gone with the 39.2 grain charge, a *slight* rise in pressure, brought on by an odd brass case or a hot day, etc., would have resulted in a 1 MOA deviation high and right of my POA. This is one of the reasons you may be getting occasional flyers with your handloads--the load recipe is not actually on an OCW node.
I shot a 1.5" group at 300 yards with my .308 load with the 168 grain Sierra Matchking using one shot of 43.3 grains IMR 4895, one of 43.6 grains, and the third shot in the 1.5" group was charged with 43.9 grains. (The OCW being 43.6 grains, of course). This proves the resilience of the 43.6 grain charge (in Winchester brass, by the way).
A group fired with my 30-06 at 335 yards consisted of loads all charged with 57.5 grains of H4350 pushing Sierra 165 grain Gamekings. One shot used a Remington brass case with CCI 200 primer, another used a Winchester case with that same primer, and the last load in the three shot group used a Winchester nickel case with a CCI BR primer. All three shots of this mis-matched trio came in at 2/3 MOA at 335 yards. (Update, I've since repeated this test for a five shot group consisting of two Remington cased loads, two Winchester nickel cased loads, and one brass cased Winchester load. Again with CCI BR and CCI 200 primers--again, the group broke 2/3 MOA. As an aside, the Remington cases weighed ten grains more than the Winchesters, but that wasn't enough difference to take the group outside MOA due to the pressure tolerance of the OCW load).
If you want to test the resilience of your own pet load, just load up a 1% increased charge, and a 1% decreased charge, and fire those two loads into a three shot group with the standard charge and see how it goes. In many cases, you'll find that either the low or high charge will group with the standard charge, but not both. This should at least tell you which way to go with redevelopment.
When you have finally identified what you believe to be the OCW, you should test the recipe with a low, normal (mean), and high charge at 300 yards to see if you get an MOA group. Assuming that you do, you can further tighten the group by altering bullet seating depth to time the bullet's exit with a good harmonic node on the vibration pattern.