Sorry, I have my fair share of loads shoot great at 500, but seem to unravel at 1K. The key for the op may be in shooting at 100 yards intervals and observe how and why the load falls apart, whether vertical and horizontal, or both. So, to the OP, if the rifle is an accurate one, 90% of the time it is the load, make some changes, one at a time. The further out you do load dev, the better chance of it holding goes up considerably.
I could type for 2 hours with examples, but one of my most perplexing ones was a brass brand switch on a 6 SLR. A few of us built them, all using junk Win orange bag brass, we all had hammer loads, hot and shot extremely well. We were ruining brass in 2 firings, yes that stupid. Local smith finds bulk Rem 243 brass at a distributor, oh boy, now let's switch. So I, do the math, case cap comparisons, blah blah, get close in theory, not much work, off and running, groups were under an inch at 500, bingo, big mistake not shooting it farther. On my next 3 outings, hitting plate from 700-1K is a crap shoot, now I am lost, finally group it at 1K, whoa, it is bad, 14-15" bad. Light bulb comes on, need a test between brass, so I did one, differing pressures on each brand, there was just no way the Rem brand was going to work once the standard had been set. Back to Win brand just to toast the barrel. This example is an extreme, which I know, but things can and do happen, and truthfully, I am not smart enough to figure it out.