Stock shape and POI

[distantfoe]

I'm running a 338 edge that weighs ~16lbs on an a5 stock and it's a shooter. I have a another 338 built on an a3 that weighs ~14lbs. I've always shot the a5 well but the rifle on the a3 tends to string vertically. Could the shape of the stock, riding on the rear bag, be my issue??

 

[6.5x47]

At what range are the groups stringing up and down?

 

[distantfoe]

I've shot groups from 100-1000 yards. It becomes exagerated at 300+.