Originally Posted by Edd
The basis of the question was using the same bullet.
I'm interested in some more info about this too.
think about load development, working up a charge weight specifically. How often do you see every charge carry the same POI? I've seen it a few times, but usually some will be higher, lower, right, or left.
To illustrate what happens when your primer goes off till the bullet leaves the muzzle. Take a wire coat hanger straitened out and set it on a table edge with 3/4 of the hanger off the table and tap the end of the hanger with your finger. That's what your barrels does, just to a far lesser degree. So if you change one variable about your load, your bullet will leave at a different point in the circle your barrel makes in it's vibration pattern, not to mention that you could also create a different vibration pattern.
Keep in mind the animals we shoot for food and display are not bullet proof. Contrary to popular belief, they bleed and die just like they did a hundred years ago. Being competent with a given rifle is far more important than impressive ballistics and poor shootability. High velocity misses never put a steak in the freezer.