In accordance with one aspect of the present invention there is provided apparatus and a method for converting circular motion to radial motion. A cap portion for covering the side of the squib with a gas injection port comprises a cylindrical side portion and a front end wall portion, and the operating pin is projected from the center of this front end wall portion. Preferably, the antigen is an Fc Receptor and the method reduces the binding of a detection antibody with Fc receptors present on the surface of many cells.