Richard Harrow is an ex US Army sharpshooter who on meeting Jimmy Darmody gets swept into the life of being a gangster.
His character is complex, he seems very sensitive, plus he is loyal and caring to friends, including the Darmody family in Season 2, and even Gillian Darmody to an extent in Season 3.
This contrasts with the fact that he seems particularly cold blooded and ruthless when performing his duties as a hit man. Throughout Series 2 and 3, we regularly get to see Harrow without apparent emotion shoot others, either from a distance or even at point blank range. His skill is a level above the average gangsters in the series, and he seems to exercise that skill in a cold blooded way, not appearing to fear death, or acting out of anger like other gangsters.
When we see Harrow prepare his weapons at the end of the episode before the finale, it seemed clear to me that he would try something like this - and that he would not hesitate to carry it through until either everyone he faced or himself was dead.
He does nearly come unstuck when he ends up grappling with one of Rosetti's men, and then he could easily have been shot as he appears to put the rifle down in the stand-off with the gangster holding the boy. The raid could easily have ended with his death.
I think we are to imply that he really is that good - but that he did have one or two lucky breaks too.