Particle simulations of drift‐cyclotron‐loss‐cone modes in a tandem‐mirror end cell confirm that sloshing‐ion distributions and the concomitant trapping of low‐energy ions by a depression in the ambipolar potential lead to improved microstablity. The simulations study the influences of neutral‐beam‐injection angle, the magnetic‐mirror ratio, the size of the depression in the ambipolar potential, and warm streaming plasma on both plasma confinement and drift‐cone instability. A simple nonlinear theory of velocity‐space transport predicts wave amplitudes and end‐loss rates at steady state that are reasonably consistent with the simulation results. This study supports both the basic strategy for sloshing‐ion operation of tandem mirrors and corroborates the first measurements of ion‐cyclotron modes in the TMX‐Upgrade experiment at the Lawrence Livermore National Laboratory.