Its upper end is drill pipe box or union, the nether end is casing pin. It is a crossover with hollow inside.

它是个上端为钻杆母扣或者由壬，下端为套管公扣，内空心的转换接头。

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A coil compression spring is captured to the pin in the hollow core of a segment and urges the adjacent components together.

卷曲的压缩弹簧被固定在段的空芯内的销上，并促使相邻的部件互相靠近。

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The built-in float pin rod comprises a hard and a soft slide rod, and moves only in the hollow holding handle, the bending spring, and the hollow casing for the rod body.

内置式浮动销杆又由硬体滑杆和软体滑杆组成，浮动销杆只能在空心的握柄、弯力弹簧和空心的竿身套管内移动。

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The government may simply insist that 3D printer manufacturers install software that prevents printing of a hollow cylinder of a certain size, a firing pin, or a certain type of spring.

FORBES: Five Reasons Gunmakers Have Nothing To Fear From 3D Printed Guns