Group going down

[Capt. D]

First; my rifle is a 7LRM. Bighorn action, 1 in 9 twist Bartlein barrel finished at 26" including brake, Mcmillian HTG adjustable stock and topped with a NF 5.5-22X50.

The components I am using are Gunwerks brass, CCI 250 primers, H-1000 powder, and Berger 180 Hybrids.

I started load developement seating the bullet at .015" off the lands. Powder charge weights ranged from 68 grains working up to 72 grains with no pressure signs. Charge weights from 70.5 grains to 72 grains all shot group sizes in the mid to high .4's, however the 72 grain group dropped about one minute (maintained group size but dropped 1") so I stopped there.

Today I seated the bullet .005" off the lands and loaded charge weights ranged from 70.5 to 72 grains in half grain incriments. The 70.5gr 3 shot group was .092" (has to be the rifle, not me) the 71 grain 3 shot group was .352", the 71.5 grain group was .453", and the 72 grain group was .658" with an ever so slight ejector mark. All of the loads were shot with virgin brass. I do not have a chronograph so I don't know the speeds.

Don't get me wrong, I love the rifle. My question is, with each powder charge increase the group as a whole moved down approximately .250 moa. Is this a barrel node thing or something else. I let the barrel cool between groups and made sure my front rest and rear bag were located in the same place each shot.

Thanks

 

[MagnumManiac]

Obviously, your barrel harmonics are changing with each charge weight. The bullets are exiting at different points in the barrels movement.
I NEVER take notice of POI during load development, only group sizes.
I would use the .092" load and load 5 rounds at intervals of .005", .010", .015" and .020" from the rifling with the SAME powder charge, only change one parameter at a time. If you're not satisfied, take the most accurate SEATING DEPTH and load up 5 rounds at .3gr intervals for 3 lots above and below your most accurate load. This would be 69.6gr, 69.9gr and 70.2gr below your most accurate load and 70.8gr, 71.1gr and 71.4gr above. Shoot each lot for groups, do not worry about POI changes, only concern yourself with the group size.
Only change ONE PARAMETER when looking for the most accurate load.
There is the OCW method or Ladder Test, they are different techniques that result in the same thing, accuracy nodes that have a wide band for velocity differences.

Cheers.
gun)

 

[Capt. D]

I certainly can't say that I am dissatisfied with the rusults of the 70.5 or the 71 grain load and not sure that I will be able to duplicate the 70.5 grain group but sure need to find a chronograph. Being 1 1/2 grains shy of the recommended load makes me curious. Also I'm sure that pressures and velosity will change once I start loading once fired brass, not virgin brass due to a little more volume in the fire formed cases. Order a magneto speed is definately on my list of things to do. I will certainly keep playing with seating depth to see if I can get the higher charge weights to tighten up, then start putting some distance behind it.