Why did changing buffers change my POI?

[lazyengineer]

Did you mean to say “carbine buffer” inside a National Match lower?

No, but - I can see the confusion. But naw, I was trying to run NTIT drills (rattle battle) which are 30 rounds in 50 seconds on 600 yard targets, and that will smoke a National Match upper. Especially if you do 3 runs. So I tried being clever on a lark, and took an Carbine upper with a "who cares" Barrel. And dropped it on the lower. So practicing on my trigger and stock still, at least. And do practice on that barrel. It wasn't a good idea regardless,but I sometimes will run stuff on a lark. The (zero'd) carbine upper zero was way off, in that config.