The separation of gas and liquid is a topic of general interest in science and engineering, for which tremendous efforts are endeavored to develop separators with high efficiency and reliability. However, when it comes to ultralow gas content, some of the separators fail. A special separator finds the potential herein, which consists of a swirl vane, a swirl chamber and a recovery vane. When the gas–liquid mixture goes through the swirl vane, bubbles will be concentrated into the swirl chamber center and evolved into a gas core. However, two key issues need to be addressed for the axial separator before industrial applications. One issue is that all the bubbles must be captured without exception, that is, the separation efficiency is close to 100%. The other issue is the stability of the gas core, the shape of which is sometimes rectilinear and sometimes spiral. A successful separation relies on the rectilinear shaped gas core. Focused on the two issues above, tremendous experimental, numerical and theoretical investigations have been carried out. In this article, the key challenges and solutions for the two issues are summarized.