What is the scope height above the bore? That is important. I too thought I had "cant" under control until I permanently installed a level on the scope. Adding the level made a world of difference past 500yds.
Were you feeding the ammo out of an internal magazine? If so try again feeding rounds one at a time by hand.
I have a Cooper model 52 Classic in .270wby mag. In a shooting session using hand loads that I knew were tack drivers. Here are the velocities I recorded for five 5-shot strings fed from the magazine in the order of firing. Firing from a shaded shooting position, 63F with no wind. Allowing 15minutes between shots with the bolt open to cool down.
I was using 71.9gr IMR7828 CCI Large Rifle mag primers, Swift Sciroccos 150gr. I believe that at the time Norma was making the brass for
Weatherby.
1. 3340, 3345, 3351, 3375, 3418
2. 3335, 3348, 3357, 3373, 3424
3. 3337, 3347, 3359, 3378, 3431
4. 3342, 3344, 3366, 3382, 3404
5. 3343, 3348, 3361, 3382, 3416
This test resulted in commensurate changes in POI and group size.
The next five 5-shot string data set was hand fed one round at a time.
1. 3340, 3345, 3351, 3342, 3333
2. 3338, 3348, 3340, 3348, 3334
3. 3332, 3347, 3339, 3333, 3431
4. 3338, 3344, 3336, 3342, 3334
5. 3343, 3338, 3341, 3352, 3336
Grouping was less than 1moa at 200yds.
Looking closely at the data you'll see that in the first group each shot of each string is continually increasing
Each shot in the second group moves around with a very reasonable ES and Mean which did show itself in
the on target performance.
I again started a five 5-shot test feeding from the magazine. I measured the OAL of each round and recorded them in the order I put them in the magazine. This time I fired a round. Then pulled each round out of the
magazine and measured each one with a dial micrometer and recorded the lengths in the order they were removed from the magazine.
Then I put them back in the magazine in reverse order, fired another round and measured again until all 5 rounds had been fired.
I quickly found that rounds that were all the same OAL when I started were getting progressively shorter with each shot. Recoil was moving the rounds in the magazine driving the bullets against the front of the magazine and back into the case. This caused the internal volume of the case to be changing with each shot. The lessening volume increased pressure increasing velocity. These were 3x fired cases. I had not previously crimped but let friction do its job. From then on I annealed and crimped. I've seen this on a number of calibers with stout recoil. The .270wby mag is a belted magnum.
