Richtmyer-Meshkov experiments for a membrane-less, sinusoidal gas interface are carried out in a vertical shock tube for a pre-shock Atwood number (A = (2 - 1)/(1 + 2)) of approximately 0.68 at M = 1.26 and M = 2.05. The perturbation amplitude is obtained by analyzing a time sequence of pre-shock and post-shock images. The Mikaelian and Dimonte & Schneider models both predict the observed growth in the perturbation amplitude, with better agreement obtained for the data at M = 1.26.