We introduce a mechanically constrained, 3D ultrasound- guided core-needle breast biopsy device. With modest breast compression, 3D ultrasound scans localize suspicious masses. A biopsy needle is mechanically guided into position for firing into the sampling region. Th needle is parallel to the transducer, allowing real-time guidance during needle insertion. Lesion sampling is verified by another ultrasound image after firing. Two procedures quantified targeting accuracy of this apparatus. First, we biopsied eleven breast phantoms containing 123 embedded, cylindrical lesions constructed from PVA-C (poly(vinyl alcohol) cryogel) with diameters ranging from 1.6 to 15.9mm. Identification of the colored lesion in the biopsy sample and analysis of the post-biopsy US images provided a model for the success rates. Using this, we predict that our apparatus will require six passes to biopsy a 3.0 mm lesion with 99% confidence. For the second experiment, agar phantoms were embedded with four rows of 0.8mm stainless steel beads. A 14-gauge needle was inserted to each bead position seen in a 3D ultrasound scan and the tip position was compared to the pre-insertion bead position. The inter-observer standard errors of measurement were less than 0.15 and 0.28mm for the bead and needle tip positions, respectively. The off-axis 3D 95% confidence intervals were determined to have widths between 0.43 and 1.71mm, depending on direction and bead position.