Modifications to the electrothermal plasma gun SIRENS (Surface Interaction Research Experiment at North Carolina State University) have been completed to allow for acceleration experiments using plastic pellets. The barrel is assembled from separate sections of 15 cm each, such that the acceleration path could be varied from 15 to 60 cm to study the effect of ablation and viscous drag, optimize the performance of the gun, and to provide longer acceleration paths for longer pulse lengths. A diagnostics system for velocity and position of the pellet is installed, which includes a four-branch break-wire measuring array situated at various locations along the acceleration path. A pulse forming network (PFN) was used to provide variable pulse lengths for the experimental shots. The longer pulse length allowed the pellet to accelerate for a longer period of time and thus reach higher exit velocities. Pressures of 100-600 MPa can be achieved, depending on the pulse duration and input energy to the source. Modifications have been implemented to the 1-D, time dependent code ODIN (One Dimensional INterior code) to include pellet friction, momentum, and kinetic energy with options of variable barrel length. The code results in the new version, POSEIDON (Pellets On SIRENS Experimental Device modeled One-D), compare favorably with experimental data and with code results from ODIN. Predicted values show an increased pellet velocity along the barrel length, achieving 2 km/s exit velocity. Measured velocity, at three locations along the barrel length, showed good correlation with predicted values. The code has also been used to investigate the effectiveness of longer pulse length on pellet velocity using simulated ramp up and down currents with flat top, and triangular current pulses with early and late peaking.