The goal of this experimental research is to determine the potential of pure acoustic detection for hostile fire indication for helicopters, while guarantying a very low true false alarm rate in the case of high helicopter speed, large helicopter bullet distance range and small calibres. An integration into a multi-sensor system, could provide a fully embedded, defence-feature technology capable of immediately alerting a helicopter crew that even a small projectile has closely passed by their machine, and provide an indication of the bullet trajectory. This article summarizes the design, the test in real-flight situation, and the performance results of an experimental acoustic array demonstrator developed for this purpose and evaluated for a referential of situations of 4 small arms firings with calibres from 5.56 mm to 12.7 mm and a total of 592 firings 52 flight exposure situations (altitude, hover, speed, orientation). The detection system is an experimental planar, centimetric, acoustic array recording unit designed for low sensitivity to laminar and turbulent air flow, rear pressure and vibrations and high acoustic selectivity. This is combined with a transient signal post-processing solver that is implemented as a deterministic cost minimization function. The conclusion of the research is a strong confirmation of the feasibility of an under-helicopter acoustic detection of small calibres hostile firings as well as the proposal of a general performance law for the capability of such a system to detect small calibres, including passing-by distances of up to several hundreds of m and speeds of up to 120 kn.