Seafloor sediment acoustic returns are comprised of scattering from both the sediment-water interface and the sediment volume. For low frequencies, volume scattering is often the dominant mechanism; however, direct measurements of this component have rarely been made, owing to the large beamwidths typically associated with low-frequency echosounder systems. When beamwidths are large, the sediment interface and volume returns arrive at the same time, causing estimates of volume scattering to become biased by the interface scattering. Endfire synthetic aperture sonar (EF-SAS) can achieve narrower beamwidths by coherently combining multiple acoustic returns as a vertically oriented transmitter and/or receiver is moved toward the seafloor. This article will present a proof of concept for EF-SAS, which will include an analysis of EF-SAS processing methods and an experimental verification of EF-SAS beamwidth improvements. Results from a motion-controlled EF-SAS experiment indicate that EF-SAS gains can be accurately predicted and that these gains are significant enough to remove the interface scattering bias.