I was just curious if you can install a regular barrel into an upper that has M4 cuts??
I know the other way will not work..
Thanks

Yeah - just think about what the ramps will look like.  They'll stop on the extension before they get to the receiver, then the receiver will have ramps that lead up to the "dead end" of your extension's ramps.  You'll have a little pocket there between the extension and the receiver on each side to catch any bullet tip that hits them.

It'll probably work, because people say their rifle/standard ramps work just as well as their M4 ramps on semi-auto, so the bullet tips will most likely always hit on the ramp.  However, if your mag spring gets weak, or if something else goes wrong, you could put a bullet tip in one of those pockets and jam.

You could just use a dremmel or chain saw file to cut the feed ramps on the barrel Xtension till it matches the receiver perfectly.