Why did McGinty believe in McMurdo?

McMurdo is clear that he is loyal to McGinty. McMurdo also likes McGinty because he is intelligent and tough.

McMurdo has shown McGinty a flash of his fake criminal history and makes himself a very professional person in crime to be counted in Mcginty"s most loyal and trustworthy men.

As when mcginty pulled mcmurdo to the side he pulled a gun on him and he stays calm and he also shows mcmurdo his criminal history which he doesn't expose him