Rifle A Lot 1
Rifle B Lot 1
Rifle A Lot 2
Rifle B Lot 2
Rifle A Lot 3
Rifle B Lot 3
Rifle A Lot 4
Rifle B Lot 4
Rifle A Lot 5
Rifle B Lot 5
Comparison table
Rifle A and Rifle B are both very good benchrest rifles, both built off the same actions, with nice benchrest stocks, shot off 1-piece rests, testing 5 different lots of Lapua Center-X. The rifles shot them best in this order:
Rifle A
Lot 3: 0.199 MOA (100%)
Lot 5: 0.219 MOA (110.1%)
Lot 2: 0.224 MOA (112.6%)
Lot 1: 0.236 MOA (118.6%)
Lot 4: 0.255 MOA (128.1%)
Rifle B
Lot 5: 0.252 MOA (100%)
Lot 1: 0.263 MOA (104.4%)
Lot 4: 0.267 MOA (106%)
Lot 2: 0.275 MOA (109.1%)
Lot 3: 0.291 MOA (115.5%)
Rifle A avg: 0.227 MOA
Rifle B avg: 0.270 MOA (19% larger)
All shot on the same range on the same very nice day with conditions that changed very little. Both rifles tend to shoot better than this when fed Eley. Zero of this ammo bought for these rifles after the test because of that. We had a chance at a nice group buy so we all tested these 5 lots of Center-X in our various guns. I could be wrong, but I only recall one of us buying any of it. Shot really well in one of our guns, and they bought a bunch of that best lot for that gun.
You are correct in saying that there are a lot of variables within the ammo itself that account for differences in performance from lot to lot. There are also a lot of variables in each gun itself that account for differences in performance with a given lot of ammo. The fact that a given lot of ammo is consistent doesn't mean it is going to shoot well in a given gun. The fact that a given lot of ammo gives consistent MVs in a given gun doesn't even mean it is going to give similar MVs with similar consistency in another gun. Variables within the gun itself can change the consistency of MV of that ammo. Even two guns built off the same model of action in similar stocks with similar barrels shot off the same rest can and likely will show considerably different results with the same ammo. Every gun is a little bit different. And its unique characteristics get along better with ammo that has certain complimentary characteristics. That's why lot testing is a thing. And that's also why brand A and brand B aren't necessarily going to resemble each other in performance when fed into a given gun. In other words, they have preferences.
I've also got examples of some ammo shot through two or more different rifles that show the same lot of ammo having different BCs depending on which rifle they were shot out of, as well as different MV SDs and ESs. A given lot of ammo doesn't perform the same from rifle to rifle in more than one way. The groups can show significant differences. The MV stats can show significant differences. And, yes, even the BCs can show significant differences because of how the barrel affects the bullets. That means one rifle can show more resistance to wind than the next even when fed the same diet. What comes out of the muzzle isn't the same from rifle to rifle, even when pulled from the same box.