Handloads that group well at 100 or even 200 yards may not group at 300+ yards.
In developing my long range .308 load, the first thing I did was upgrade my components. I switched from whatever brass, to Lapua .308 brass, Sierra 175 Gr Matchkings, and Federal 210M primers.
Combine that with even more attention to detail in the loading process, and I was able to work up a load that shoots well under .5 MOA. It will easily group at 3 inches or less at 600 yards.
As a bonus, it is not by any means maxxed out. The load i ended up with is 1/2 grain under the book max. The book max load showed some pressure signs, and did not group as well.
Ultimately, find a load that groups well at 100, and test it at 300 or more yards. If it's consistent there, you are onto something. Also, don't be afraid to load the round a little longer, just a little bit off the lands/grooves of the rifling. Minimizes bullet jump, and can make a difference in accuracy. You can have a gunsmith find the optimum length for you, or you can just load a dummy round, and seat the bullet down little by little until the round chambers correctly.
VIGILIA PRETIUM LIBERTATIS
"The price of liberty is vigilance"
America is at an awkward stage. It's too late to work within the system, but too early to shoot the bastards.