In this paper, we investigate an efficient scheme for data multicasting in narrowband public safety radio systems. The proposed scheme uses fountain encoding, in order to avoid feedback messages for each individual packet. We first propose a multistage estimation scheme that is based on slotted random access and that does not require any prior knowledge of number of users. The rest of the proposed scheme consists of iterative transmission and feedback/estimation phases. In feedback phases, the base station estimates the number of users that did not decode the data file yet, and determines the number of additional fountain encoded packets required. This phase does not require any channel state information. The proposed scheme lasts until all the users in the group decode the message. We show that the proposed estimation scheme is accurate and computationally efficient. The overall multicast scheme performs very close to an optimistic lower bound, which makes it a promising multicast approach in public safety radio systems such as APCO P25.