cause of low shots?

[rufous]

I am having some problems with my 300 Win Mag. I had a great load worked up for it last August using the Nosler 200 Accubond and RL25 powder. Then the accuracy of that load dropped off. Not sure why. Anyway I have been getting a lot of vertical in my groups, no matter what I try. I have come to the conclusion that it is either me and the way I am holding the rifle (follow through, cheek weld, amount I am pulling the rifle into my shoulder, placement of buttstock on my shoulder etc) or the barrel is moving in the action threads. Both options are hard to believe. The load I worked up in August averaged 0.59 MOA for several groups at 250 to 400 yards but now no matter what I try I keep getting vertical fliers. The barrel moving in the action threads seems unlikely as well because Greg Tannel did the gunsmithing and he uses the Spirallock thread design which is supposed to prevent that from happening.

Anyway if it is my doing what am I doing wrong? When I get a vertical flier it is much more often low. For example today I shot prone at 250 yards with my bipod and a sandbag under the buttstock. These 9 shots were fired with the 200 grain Nosler Accubond seated 0.005" into the lands and 79 grains of H1000 powder. First 3 shots were in 0.74". After cooling I fired another 3 shots. The first shot of the second group landed in with the first 3 shots but the next two shots were lower (shot 5 was 3" lower and shot 6 was 2" lower such that all 6 shots were in 3.9" with 5 of those in 2.48"). Then after cooling I fired the last 3 shots. The first shot of the last group was 3" low also and 2" to the right. The 8th shot was 1" low and 1" right of the first 4 and the last shot went in with the first 4. So I had 9 shots in 4.5" at 250 yards with 7 of those in 2.48" and 5 of those in 0.79". I kind of had two groups. One group was 5 shots in 0.79" and the other was 4 shots in 2.8" but it was lower. Again if I am causing the low shots what am I likely doing wrong? Thanks for the help, Rufous.

 

[a.JR]

Rufous,Take the gun to the gunsmith and get the crown inspected,could be worn or damaged,if it is get him to recrown it and go back to the original best load..JR..Jeff Rogers

 

[Sako7STW]

I doubt this is the problem but I had a gun that did this once. The problem ended up being a broken crosshair in my scope. I would go out and shoot and it would do great then all of a sudden i would have one that would be way to the left or the right but still in the same verticle plain. Turned out the verticle crosshair was swinging back and forth. I found out the worst way possible that something was wrong, I hit my antelope through the hind quarters. I chased him well over 2 miles watching him dragg his hind quarters. I finally got him thank goodness. I felt so bad and was so upset that I almost quit hunting period. Not a good start for hunting, that was my first hunt. We checked the gun when I got home as I swore I had held right and sure enough we found it.