Are CMMG .350 Legend magazines compatible with other manufacturers uppers/ARs?

The short answer is yes, but depending on whose upper/AR you are wanting to use there may be some work needed to ensure proper function.

After some research we discovered that other popular .350 Legend AR manufacturers barrel extensions and magazines all differed slightly from one another, including ours. The simple fix to avoid feeding issues (if you have any) is to simply file the front opening of our magazines slightly to provide enough clearance for the round feed properly.