This work is a risk-reducing phase before a possible manufacturing of a gas generator for RBS 12. The propellant consists of HTPB as a binder and ammonium nitrate as an oxidiser. The crystal phase transformation of ammonium nitrate is a complicated process, which may give severe problems when storing the propellant. Ammonium nitrate is also hygroscopic. A type of granulated ammonium nitrate from Hydro Chemicals, called AN-K, has been experimentally studied. Rapid water absorption and clogging of AN-K does only occur in air when the air humidity is high. The weight increase is approximately 1% at 68% RH. To obtain easily cast propellants with a content of approximately 78% AN-K, some of the granulated material must be milled. Propellants with ca 79% AN-K have been manufactured and environmentally tested. They endured reasonably high temperatures and humidities as well as several temperature cycles without absorbing water or undergoing shape changes. The results indicate that AN-K is well suited for use as an oxidiser in gas generator propellants. Further studies concerning especially burning velocities and mechanical strength for environmentally tested propellants should, however, be performed before the AN-K material finally can be recommended.