A Thomas Turner 577/500 #2 BPE measures .185 just forward of the rim recess with a steel barrel.
The hammerless Henry 577/500 Mag. measures .185 also but with a damascus tube.

We'll need ot wait until either Friday or Monday for delivery of the other Henry. I called and they just shipped it today.

edit:
The Hammer/underlever came in today and it's measurement is .200.

Now I'll need to get pictures up before Mehul starts in on me!