I have several hunting rifles that have never fired a single round of factory ammunition. I've always reloaded for them.
When shooting M1 Garand in service rifle matches I was required to use M2 ball ammunition. I remember doing a couple of test targets at 100 yards to compare the issue ammo to my own reloads.
With the GI issue ball ammo (LC 1960's production) my Garand will regularly shoot groups of 3 inches at 100 yards. With my practice ammo (reloads, 147-grain FMJ boat-tail surplus bullets, segregated case lots, trimmed, full-length resized, etc) the same rifle will regularly shoot groups of 3 inches at 100 yards.
This is an unaltered, as issued M1 rifle (Winchester 1943) in well-used condition.
It appears that the best that I can do is to reload ammunition that is equal to what was produced in huge quantities for the military.