My friend Mal first made me aware of this and Sonny in Wyoming has used a variation of it in his reloading.
First of all, credit to the guys who first developed and put this information out there.
Dan Newberry – http://optimalchargeweight.embarqspace.com/
Christopher Long has produced a detailed theoretical basis to Dan’s method in his Shock Wave Theory document, http://www.the-long-family.com/OBT_paper.htm.
My simplified summary of these gents work is as follows.
When a cartridge is fired in a rifle there are a series of pressure waves that pulse up and down the barrel while the projectile is still in transit up the bore.
These pressure waves cause a slight wiggle in the end of the barrel. If the projectile exits just as the barrel muzzle is doing its periodic wiggle then there will be a slight random variation in the bullet’s trajectory.
Varying the powder charge in the cartridge causes variations in the pressure of the detonation and this effects the muzzle velocity of the projectile. Ideally, we want the bullet to exit while there is the least wiggle in the barrel muzzle.
The optimal charge weight (OCW) is the amount of propellant that causes the projectile to exit during one of these periods of relative stable muzzle conditions.
The OCW method is designed to find one, or more, of these sweet spots when the barrel muzzle has the least vibration. Loads that achieve this are OCW loads and are very consistent and reproducible. Note that they may not always be the most accurate load.
Having found the OCW load you can then vary projectile seating in an effort to improve on group sizing. Again, it is noted, that often there is little impact on group size by varying seating depth once you are at an OCW loading.
The OCW is found by plotting the centre point of each group. Typically, during load development, as propellant charges are gradually increased and the resultant groups measured, there will be a bit of a wander in the group centre point.
When three or more groups have pretty much the same centre point then this is indicative of an OCW loading. The middle load is taken as the actual OCW and further testing can be done in small steps around that point. This is where seating depth can also be varied.
I have mapped out a test load sequence for my 223 Rem. If that works okay then I will do similar with the 257 Weatherby Magnum.
The problem at the moment is a lack of projectiles. I waiting on stock out of the USA, where supply of shooting components seems to have slowed to a trickle.