I just put the XS Big Dot on my 3" SP101. I noticed that the white ring of the sight fits nicely in the rear sight slot with very little space between the white ring and the sides of the rear sight. When the Big Dot is nestled properly in the rear sight (bottom of the white ring just touching the bottom of the rear sight slot and centered in the slot) the tritium dot in the center of the Big Dot lines up with the top of the sides of the rear sight. This is the same sight picture as with a blade front sight. I haven't fired the gun with the Big Dot yet but it looks good.