Apparently someone else in that club did have a gun. First reports in the news were that an off duty police officer was present, with one mention that he was club security. He returned fire and caused Mateen to retreat to that bathroom with hostages-that is- he took hostages rather than killing them. He would not have done that did he not face another gun. The officer probably prevented further fatalities as the hostages were not killed by Mateen and he had a restricted field of fire from that bathroom.
Read the same. However, the monster still shot some in the bathroom according to an eye-witness account on Fox News. If there were more people carrying, I believe they could have taken the muslim sh*t out before retreating to the bathroom.