Why did changing buffers change my POI?

[lazyengineer]

Actually I'm glad we have this thread. I once put a carbine upper on a National Match leaded lower, and found it's POI shifted A LOT. As folks here say though, there are lots of factors in play, ranging from how the gun was being adressed (I was running National Match rapids, and wanted to burn that BBL in practice, rather than my good 20", for some tests), and it doubled the weight of the gun, meaning the mechanical locked-system recoil was different, before the now rifle-buffer ever got touched. But really, it probably had more to do with my significant difference in addressing the gun running a leaded A2NM lower in prone, than a light weight M4 lower from teh bench - is my guess based on the replies. I always wondered, but never chased it further than that.