A biopsy instrument for use in a biopsy gun includes a hollow outer needle defining a lumen therethrough and an inner needle slidably engaged within the lumen. The inner needle defines a cavity adjacent its distal end for receiving a tissue sample. A first hub is attached to the proximal end of the inner needle, and a second hub is attached to the proximal end of the outer needle. An elongated pin having a first end and a second end is fixedly attached to the first hub at the first end of the pin. The second end is in sliding contact with the second hub so that the second hub and the pin are slideable relative to each other. A stop engagement is provided between the second end of the pin and the second hub and is configured to prevent disengagement of the outer and inner needles and to maintain a specific physical relationship between the hubs.