When done in the barrel itself (fixed chokes), a flared muzzle end was often termed a Tula Choke like this set of Krieghoff barrels.
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But I have never seen this on a Win SX-2. There are a number of pics of SX-2 barrels on the net and they are all straight as far as I can tell...like this
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Is this a fixed choke or screw choke barrel....??? I thought they were all screw chokes which sort of eliminates Tula chokes, I believe.