Tunneling is often used to describe multiphoton ionization of rare gas atoms in infrared fields. We test the tunneling approximation and its nonadiabatic extension by measuring the unperturbed momentum distribution along the κ direction of a circularly polarized light pulse. We find substantial, but not total, agreement between our results and the predictions of the model. As predicted, the κ direction momentum distribution is Gaussian and its width increases with the square root of electric field strength. However, the width is 15% too large and we find no evidence of nonadiabatic effects as we approach the expected limits of the approximation.