I have some high end custom rifles that shoot many Factory loads like crap, 3-4 inch groups
BUT find what it likes, and they will shoot 1/2' MOA
a lot IMO comes down to finding the load YOUR gun likes
and in factory ammo, there can be variations that can cause SOME smaller/larger groups out of same box, there NOT as precision reloads as one that is looking for extreme a curacy from there gun/ammo!
and why most all true long range shooters(not being sponsored by an ammo co)
will be using reloads in matches!
I fully agree, a LOT has to do with the shooter, its about being repetitive in your hold, aim and shooting form in general!

I however am NOT a huge fan of super high magnification scopes, at higher magnification I get too much mirage and not fan of that, all the more so on a HUNTING rifler and NOT a match rifle
as in life pro's and con's to all things
a 1,000 yards is a LONG way out, ad a TON of things have to come together to make things into small groups
and countless variables are beyond the shooters control!