The M4 ramps extend down into the upper itself due to the different angle.
Randall at ar15barrels.com has a good picture on the difference (see link below):
http://ar15barrels.com/gfx/feedramps.jpg
I have gotten away using an M4 cut extension on the barrel and standard receiver and an M4 reciever and standard barrel with out problems. BUT this is unusual (or lucky), in any event I would makes sure the barrel extension is cut for the upper you will be using...This helps to take one more potential problem out of the loop.
Just one mans thoughts...
Happy
DonT
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