Many of the Mossberg bolt action shotguns used a removable choke, not an adjustable. Sold under the name C-Lect-Choke, this was an external choke that could be unscrewed with a special wrench, and a different choke screwed into its place.

C-lect choke is an adjustable choke system that Mossberg implemented on their shotguns pre-1977. After 1977 they went to the accu-choke system. The choke is adjustable between Full, Modified, and Improved Cylinder. It is, however, not recommended for use with steel shot.

-- Adjustable Choke -- Unfortunately, the adjustable choke is almost certainly an add-on which greatly hurts any collector value the gun may have had. In typical condition for a pre-war A5, with the added choke, value is around $350-$450. sales@countrygunsmith.net

You don't give us the model, so it is hard to answer the question. Assuming the gun does not have an obvious adjustable choke, the choke is probably not easily adjustable. A gunsmith can open the choke (from Full to Modified for example) or can install internal interchangeable choke tubes (costs around $125).

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