An adjustable coaxial needle assembly for placement into a tissue mass prior to a biopsy procedure comprises a guide cannula assembly, a stylet, and a throw calibrator. The stylet is selectively coaxially received by the guide cannula, which is in turn coaxially received by the throw calibrator. The effective length of the guide cannula can be selectively adjusted by moving the throw calibrator relative to the guide cannula to adjust the distance between an open distal end of the guide cannula and a the throw calibrator.