Thread: Trying something, Eric Cortina method of load development

Originally Posted by GWH

Yep seating depth has a huge effect on grouping. As far as im aware its all to do with at which stage of the barrel harmonics that the bullet is exiting the muzzle.

From the seating depth testing ive done with various rifles and loads, the difference in grouping can be considerable.

My most recent test (last week) with 65gr SGK's in a model 7 223. I loaded several groups, starting at .005" off the lands and going to .030" in .005" steps. All but one shot at .6-.7 moa, .015" off the lands shot .25 moa.

Id already worked out what powder charge to use from ladder tests, and some group testing at whatever coal i started at.

Clearly this particular bullet in this barrel isnt that fussy, ie .6 moa isnt too shabby, but i'll take less than half of that any day if its available just by tuning seating depth.

Ive loaded up a bunch more at that coal and charge to test further to make sure it wasn't a fluke.

Yup Cortina states any load can be tuned by changing seating depth and shows targets shot showing where the nodes begin, peak and taper off.
He also states that there may be multiple seating depth nodes and dome are larger than others so you need to do a comprehensive seating depth ladder to find one of these larger nodes as they will be more stable and more forgiving to tolerances created by limitations of loading gear and to an extent the skill level of the person loading.
As previously stated I'm sure a lot of this is not new or ground breaking, just the way Cortina explains it makes sense to me.
It's really very Interesting seeing the results learning how my changes influence the results and why, also understanding how to interpret what is happening especially in the seating depth ladder.