A stable fire retardant mixture for use in a backstop for decelerating and trapping projectiles. The backstop generally includes a support structure having an inclined surface and the stable fire retardant mixture serving as a projectile trapping medium disposed on the inclined surface. The projectile trapping medium is a resilient granular material intimately mixed with a hydrated super absorbent polymer (SAP) gel and additives. Preferably, the support structure is made of a shock absorbing, foamed, fiber-reinforced concrete, such as SACON.RTM.. In embodiments, the support structure also includes an enclosure. The additives control alkalinity, chemically stabilize the mixture, prolong life of the mixture, retard mold formation and bacterial growth and prevent leaching of heavy metals.