If you are new to the OCW load development concept, it may help if I share with you an example of how it is done:
Some while ago, I developed a varmint load for a Remington 788 I owned in .243 Wincheter. The bullet was the Sierra 60 grain HP, and the powder used was IMR 3031. Primers were the CCI 200's, and brass was all twice fired Winchester.
In checking three loading data sources, I noted that 39 to 41 grains of IMR 3031 was the max charge zone for that powder 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 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 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 groups from the middle of the string were the ones which happened to hit the same POI, (all within about a 3/4" overlay, and all at a 2 o'clock position with relation to the bull) 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 just under 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 (the sub 1/2" group), 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.
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).
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. A point of interest, 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). By the way, if all brass and primers are kept uniform with this load, it is about a 1/3 MOA performer (1 inch) at 300 yards, so I'm not saying that uniforming brass doesn't help--it obviously does. It's just that an OCW load is much more tolerant of pressure altering component changes.
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.
After you have positively identified the OCW, you can then adjust the seating depth to fine tune the load for ultimate accuracy. After doing so, you can be assured the final load recipe will be a consistent, solid performer.