anyway--i have a 223 VS with full legth action bedding block. just for fun i smeared some steel bed behind the lug and under the action and it filled in the 1/16" gap running down each side of the action, that i couldnt tell were there originally. bedding that block cant hurt--let me know how it turns out.
You are jumping around changing 2-3 things at once trying to cure one item.
Stop and regroup.
Horizontal could be from several things, some of which have been discussed. I am going to assume that an equipment problem has been eliminated as cause and wind was not a key factor.
-poor stock position and handling
- seating depth.
It sounds like you originally had a load that wanted to shoot and and you started shooting different loads of .005 depth changes, that is part of the tuning process. Seating depth changes will cause the horizontal. Keep the powder where it is at originally and shoot the .005 different groups to confirm which seating depth your gun likes and then shoot 2-4 more groups at varying ranges to confirm. The right seating depth will take out the horizontal. You can change neck tension by changing bushings or amount of brass turned off neck. Look for the group to tighten up and then start to open up again. Same way with incrementing the powder charges.
By changing powder weight and depth at once you are really hit or miss and you do not know what caused it to go bad or good. Only change one thing and shoot until you know where it tightens up and opens up.
I too am having some problems with horizontal shots. I am shooting a 7mm Rem Mag with the 160 Accubond. I have tried varying the powder charge and the seating depth. I am pretty happy with the load I have come up with (8 groups averaging 0.85 MOA) but the larger groups are due to a shot off to the left or right. I was surprised by the comment that seating depth can have an affect on horizontal stringing. I had thought that powder charge and seating depth mainly affected vertical and that horizontal was mainly wind or gun handling or parallax. So far it seems that my gun is shooting this bullet best at touching the lands or 0.010" into the lands. Same basic accuracy at both. I have tried 0.010, 0.020, 0.025, 0.030, and 0.040" off the lands also. I have not tried 0.020" into the lands though. Maybe that would be even better? I did seat a bullet long (0.030" into the lands) and chambered the round just to see if the bullet would be pushed back into the case neck. It did not move. Neck tension must be farely high. I am using an RCBS competition full lenght sizing die, not the Redding bushing dies. Do you think there would be any point in trying 0.020 or 0.030" into the lands? Rufous.