【单选题】
A machiningfinsdirectlyontheoutsideofthebarrel.
B shrinkingonajacketormuffofaluminumcoolingfinsaroundasteelcylindersleeve.
C machiningfinsdirectlyontheoutsideofthebarrelandshrinkingonajacketormuffofaluminumcoolingfinsaroundasteelcylindersleeve(ondifferentareasofthebarrel).
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