This is the famous M203 Tangent Sight, designed to be used with the M203 40mm launcher. Now you can use this with your Tac-D on any AR or Picatinny equipped weapon. If the weapon that you own has a carrying handle you are good to go; it clamps right on. If you need a carrying handle, fear not, just navigate our catalogue and you will find a good example from NCStar.
The link below will take you to a pdf that describes the use of the M203 Tangent Sight in detail.
If you have an AR with an INTEGRAL carrying handle, you can order either variation though the one for the integral handle (narrow spacer) is most appropriate. If you have a REMOVABLE handle you MUST order the sight for use with that type of handle because the older type will not clear the tightening nuts (wide apacer). Read the doc below for a full and tedious explanation. Call if you have any questions.
How to use the M203 Quadrant Sight