Originally Posted by datsjeep
Ok Magnum if I understand you correctly. You start with a COL of magazine length - .005. Retumbo seems to have the highest velocity listed with 71 grains being the max. So I would load 5 shots each of 68,69,70,71
After that is where you lost me. Assuming 69 grains is the most accurate you would load 1 each between 68 & 70 in .3 grain increments and shoot them in a round robin (?) to determine where the accuracy node is ????? I dont get it.
Then you would reduce the seating depth by .005 to find a sweet spot on COL. Assuming the sweetspot on my gun is somwhere close to the federal premiums I have shot accurately, moving my COL by .005 and 5 shots each would take me 103 rounds.
I load a total of 25 rounds @ -.005", -.010", -.015", -.020" and -.025" from the mag max length minus the initial .005"-.010". That's it.
Presuming your sweet spot is the same as any factory ammo is erroneous, it may be but most likely will be something completely different.
The sweet spot is generally somewhere very close to the lands or even into the lands, and then there is a second spot further from the lands that it will be in. What it is is where the barrel harmonics are in tune with the bullet barrel time.
The OCW method is, assuming your load is 69gr's, you would load 5 @ 68.3gr, 68.6gr and 68.9gr, then you would load 5 @ 69.3gr, 69.6gr and 69.9gr, presuming you're not working with a max load of course! I shoot these randomly at the same aiming point to determine which loads shoot nearest to the centre of the group, this way helps to eliminate any 'vertical' at longer ranges, this also known as the 'accuracy node', it may be that even a 2gr spread may shoot to the same POI, which is the Optimum Charge Weight for that bullet/powder combo. (If you're not comfortable shooting them randomly, shoot as groups as you normally would in strings.)
You then work with the most accurate of these, or the ones that print to the centre of the aiming point, those that are high or low are discarded from further testing. This test should be performed at 200yrds (min) or further for best results.
In other words, a charge that is 69gr for instance may be completely stable even if there is a variation of +/- 1gr from the desired charge of 69gr's. This doesn't happen too often, a variation of 3% is typical, which equates to about +/- .5gr in most large capacity cases.
If anyone can add anything I might have missed, please do so.