Took this picture, and it seems like your measurements and what I'm getting are right around the same, Doh... I will say that with the naked eye, mine MIGHT be just a hair under 3/8ths. My only reservation about the chuck now being the problem is the fact that I've seen the same exact issue on on two separate chucks, both Dremel brand products, and both obtained at separate times. One was sent to me from Dremel directly when I first contacted them about this issue. I still have not heard back from them regarding my last email in which I explained that the newer chuck they sent did not fix the issue. I believe that was just this past Friday or Saturday.