Handloaded ammo is custom tailored to the specific rifle you are firing it in. Factory ammo is tailored to be good or great in MOST rifles.
you often can find factory ammo that will shoot excellent in your rifle, especially if you are using something like the 308 win or the 223 because there is a lot of Match grade factory ammo available.
As far as your standard grade ammo goes... case to case quality is not as it could be, seating depth varies slightly from round to round. I haven't pulled bullets and weighted the powder charge from factory ammo, but I would assume that it is actually quite consistent.
