If only one is cut for M4 feed ramp, the barrel extension or the receiver, it is better for it to be the barrel. If you have the receiver cut and NOT the barrel extension then there is a sharp ledge the bullet encounters at the end of the receiver cut.
If it is the other way around then the receiver has no cut and the bullet encounters a drop off at the joint between receiver and barrel extension (where the barrel extension cuts begin). A drop off will not affect feeding because the bullet has already been raised up enough to clear the initial receiver edge.
I would see if you can exchange the upper receiver for one without the M4 ramp cuts. Those are only needed really for the shorter carbine gas system and mostly for full auto reliability. Most barrels 16" or less will have the barrel extension cuts. Fewer barrels over 16" will have the cuts. I just bought a Palmetto State complete FN upper that has the M4 cuts in both, and BCM offers a 20" with M4 cuts in both, but that is unusual.
