Quote:
If the point of impact is significantly different, they bullets probably have a significant difference barrel time. For example, one may have less jump to the lands, which will raise pressure, or one may stick down into the powder space further, which also raises pressure. Raised peak pressure shortens barrel time, so the bullets will then exit at points in the muzzle swing that are enough different to cause the POI shift. It doesn't tell you how they will perform after the sight setting is corrected for them. It does, however, hint that at least one of them is probably not at its most accurate with that particular powder charge, and that you need to do some load development to find where that is. Dan Newberry's OCW method is a good place to start. If you have 300 yards, you can try an Audette ladder.
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In my .338 Lapua Magnum, my load pushes a 300 gr bullet around 2750 fps. It shoots bugholes at 200 yards and is 1/2 MOA at 600....under 3/4 MOA at 1,000.
My dad gave me some .338 Nosler partitions, 225 gr and at the time I had no other .338 rifle so I loaded them up for my .338 Lapua just for the heck of it. I chose a load near the middle of the recommended MAP from the manual and loaded all 20. Obviously I didn't have enough of them to do a load development so I was just using them to foul the bore.
Anyway, the velocity of these were about 2,900 fps which again is moderate for this cartridge and bullet combo using Retumbo which is slower. However, despite being a lighter bullet traveling faster, the POI was 8" low and right of my 200 yard zero with the 300 gr bullets still going slower at the 200 yard mark. (At around the 400 yard mark the 300 gr bullets will pass the 225)
The reason for this was that I was at the optimal barrel time with the .300 gr bullet and way off with the 225 gr bullet. Again, I didn't much care and didn't do a load workup for the 225's as they were designated foulers and I only had 20, but still, the 8" low and right was not because they were slower at the POI, but because of the difference in barrel Harmonics.
Now, at the time I didn't know all this, but I do now, and if I had everything tuned properly for both of those bullets, I shouldn't see much in the way of Horizontal deflection at all, and the vertical should be within a couple inches.