I tried inputting your data into a ballistics program - it got about the same results you are seeing. I can only echo what others have said, try for more measurements. Get an actual zero at 240 or whatever distance you choose. Get an actual measurement from point of aim to point of impact at 450. If possible, shoot longer and add this data into the analysis.
I get pretty obsessed about the accuracy of these measurements because it’s the old “garbage in – garbage out” situation.
I agree the tapered bases can cause problems but I have never had any problems getting a ballistic model to match up to actual bullet flight by playing with the numbers a bit. That is not the case here.
Problem here is time. I have 50 customers that want their big game rifles for this big game season so I am pretty much stuck doing that. This rifle has been a pretty rushed deal but I figure the longest shot I will get with it will be under 600 yards in the area we will be hunting so from what I am seeing shooting it so far this will certainly not be a problem.
My plan is to set up at 300 yards as SS7mm recommended, shoot some groups and dial in the rifle for a 300 yard zero. Then set up at 400, 500, 600, 700 and 800 yards and shoot on paper and take actual measurements of drop from the same zero hold. That will tell me all I need to know without any ballistic computer and then when I get back from this hunt I can really fine tune things for some longer range shooting.
Will keep you posted with the results.
As far as pics go, would love to post some but huntingpictures.net will not allow my pics to upload for some reason? The 338 AX is roughly the same length case wise as the 338 RUM but with a sharper shoulder and significantly fatter all around!!!
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Especially with a light rifle that has some recoil, the way you hold it can make a big difference. It sounds like you were in a different place with a different setup on the long shots vs the short shots. It may be something as simple as holding the rifle differently, being on the front rest differently, pushing back against the recoil differently, etc. So your POI might have changed a tiny bit between setups. Maybe you were holding the rifle in such a manor at 450 that you would have hit 2.5" high at 100 had you been shooting that distance at that time. Just a possibility.
Another is scope error. Be sure the reticle doesn't move as you adjust the parallax knob or when power is changed (if you weren't on the highest power for both). A collimator of some type (optical boresighter with a grid) is perfect to check for these things.