Please email us at techsales@opticsplanet.com regarding the problem you are having with the Aero Precision M4E1 Stripped Upper Receiver.
The M4E1 are designed to be a tight friction or even thermal fit with most barrel extension. This is a good thing for accuracy. A heat gun, torch or even hot hair dryer should get it loose enough to install and then tighten back down around the barrel extension when it cools.