I am sure that, regardless of your actual OD, you should take wall thickness measurements at the 12, 3, 6 and 9 o clock positions around the barrel opening.
You can see such a variance in wall thickness that you may be limited to an extra thin wall choke regardless of your OD.
I went through this exercise with a gun smith and my police open cylinder barrel in stock form would only support thin wall chokes.
I spoke to a fellow last week who had 10 Remington 870 barrels in 18 to 20 inch lengths available and based on od and wall thickness only three of them would support rem chokes, the rest would only take a win choke or a tru choke.