The fit between the receiver and the barrel extension on my M is definitely tighter than my XCR-L's.
You don't need to do anything special to re-attach the barrel, but you do want to make sure the barrel is seated all the way into the receiver first. If the gas block is lined up with the gas tube alignment should be ok, just get it fully seated.
When you snug the barrel nut then it should force it into alignment. You can tighten the barrel nut down without having the barrel fully seated in the receiver, and it may actually operate somewhat, although this should be obvious since there will be more of the barrel nut sticking out since it's not in the depression in the extension.