Help cutting an O ring grove

[mike-sid]

First thank you all for the informative replies.  Here are some numbers from my measurements.   Bore of barrels .736", part on tubes I want to machine the O-ring grove in is .717" .    The attached photos are of the tubes. They are .410 bore. Actual bore is .411" the smallest diameter on the tubes is .50"    So in light of all the info provided so far, I'm not sure of the optimum size for the O-ring and it's associated with of the grove.  When this is known, I'll turn down an AL bar and test out my O-ring with that.     The red part of the tubes are AL, and the shiny metal chamber sections are stainless. Looking at the tubes from the muzzle end, I want to install the O-rings on the second large diameter section.   The end view from the muzzle end show the changeable choke tubes.  When I do my part the clays turn to dust on the skeet field!