I hope it didn’t seem like we were all raining on your parade of late, but the most likely issue isn’t some rare and complex mechanical or thermodynamic issue, it’s most likely the shooter.

A 3-15x optic can be enough for 1,000yard shooting, if your targets are huge and your tolerance for precision is large. I shoot long range most weekends, and I almost always zoom into 20-25, if not 30x on the optics I have which go that high - even on big targets (like a full size 18”x30” IPSC, or 50” long, full size deer). With a 15x optic, shooting groups at 200yrds, I would want a large diamond, like that available in the Shoot-n-c 5 diamond targets. For 1000yrd shooting groups with a 15x optic, I want a target the size of a car hood, with an aiming reference of 30-40”. This is about the same target size we shoot in Service Rifle/NRA Highpower at 600-1000yrds. That would give me a good milliradian of aiming reference (half mil up and down), with a generous ~2mil visibility. You can’t shoot what you can’t see - and it’s hard to see small things at long distance. That Leupold will have a minimum FOV at 1000yrds of 77ft, meaning a 6ft target is less than 8% of your visible field. Your reticle crosshair lines will be at least 2” thick, and mirage can wiggle your image to appear 4” or more displaced. Placing your POA precisely on a small target at 1,000yrds is a big task (and by small, I mean anything smaller than about 20”).

If the Nosler’s are grouping “sometimes 1/2” at 100yrds” followed by “sometimes 1.5” at 200 yards,” then I’d say you can be happy with a factory rifle which shoots the way a factory rifle should shoot.

If your groups are growing during your session and the barrel’s not getting hot, then in my experience with new shooters, again, it’s not the rifle, it’s the shooter. Anticipating the shot, a little flinch from recoil sensitivity, a little trigger slap, driving the rifle too much instead of working NPOA, a little shooter fatigue... groups tend to grow for new shooters the more they shoot during a day.

The Barnes bullets, however, DO have a sensitivity, and if they don’t match your rifle well, I’m not surprised they shoot 3-4” at 200yrds - but they would also be 1.5”-2” at 100yrds. Barnes bullets, and any other copper solid bullets like them, like the E-tips and Hornady GMX’s, or partition bullets like the Swift A-Frame or Nosler Partition, all tend to prefer a lot of jump to the lands. @Big Uncle shared that tip with me several years ago here, because I had given up on them for lack of precision. So knowing your chamber and knowing how the ammunition fits is much more critical for the Barnes bullets than the more forgiving NBT’s or NAB’s. This might explain why the Barnes don’t shoot as well as the others, but if the Noslers are growing the more groups you shoot, or your groups are growing with range, it’s most likely not the rifle or ammo.