The S510k is probably the pinnacle of the S400/410/510 range. Gone is the regular bolt action and in its place is a side lever that is slick and quick to operate; it is designed to be so smooth, that a shooter can cock and load the next pellet whilst never removing their eye from the scope. Also, the S510 comes with a bull barrel that is aesthetically very pleasing and is devastatingly accurate, as well as incredibly quiet.