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The topic of this paper is the distribution of magnetic fields inside the rails of the electromagnetic railgun RAFIRA located at the ISL. The magnetic field pulse characteristics are measured using colossal magnetoresistance-B-scalar sensors placed at different depths inside the rails of the accelerator. During launch the muzzle velocity reached up to 1.4 km/s, the electrical shot energy is about 1.2 MJ and the projectile mass was 140 g. The obtained results are analyzed using two models based on analytic solutions of Maxwell's equations. The first model considers the 1-D magnetic field diffusion in the direction perpendicular to the rails. The second model includes convection and simulates the 2-D behavior of the magnetic field distribution in three regions: the armature, the contact zone between rail and armature and the rail behind the armature. Additionally, 2-D and 3-D quasistationary finite element models are developed using Comsol Multiphysics. Excellent agreement is found between the 3-D simulation results and the measurements of magnetic diffusion.