I am going to order a SF M951su with the tape switch, but I am not sure what length tape to get. The light will be mounted on a SF M73, and used with a Tangodown vert grip. Never used a tape switch before and would appreciate some input.  First post on this site, I think I've come home. Thanks in advance.

Measure the distance from where your flashlight tail end will be when mounted, to the top of the slot in the VFG.  This is the length of wire you will want.  

Wire length part of tape switch is measured from where wire starts at end of flashlight tail cap to beginning of tape switch.  Tape switch itself is not included in overall length.  

It's better to have some extra length rather than too short a length.  Depends on where you will mount light.  I use a 5" on my M4 RAS w/KAC VFG and it fits perfect.  Used a little bicycle intertube around VFG to keep secure.  Works for me.

You might also want to consider the click on/off tail cap.  I have one of these on my 16" and it is also a good option.  Less equipment is less to go wrong.  Hope this helps. good luck.

Calvin at Brightflashlights can help you with info if you need further.  Outstanding guy, good prices, great products, shipping fast.  

On mine I mount my SFs as far forward as possible. On one carbine I use a 6" pressure switch. On the other a button tailcap with the vertical grip. I find the latter more advantageous. I'm right handed, mount the light on the left rail and while grasping the grip with the left hand the thumb pushes the tailcap. I end up using this setup more than the pressure switch.

I got the seven inch cable for your same setup. Surefire rep said the 6 would be plenty for a foregrip setup and would have been.

Thing is, the pressure switch is not quite as responsive as I would like on the foregrip. Sometimes a delay in turning on when I want it on and sometimes on when I want it off.  I don't like the thought of having to stay off the switch at a high stress moment when I am shouldering the weapon and don't want to be seen.

With a right side mount on the M73 and a TD foregrip the tail cap switch (no cable or tape at all) is right under my thumb for momentary on or click full on (I find focusing on the correct grip to keep the light lit with pressure tape is an unecessary distraction.) The tail cap meets the keep it simple standard too.

Nothing against tape per se, I have the pressure tape on a Benelli foreend and it works great as you fingertips naturally rest on top of it.

Thaks for all your help. I ordered the 6"  and when I get some money saved up I plan on buying the tail cap that has both.  After reading your posts it may be sooner than later.  I am one of only three on my dept. that is patrol rifle certified, and though we are not currently issued rifles (total b.s.) some dark night I may need mine, and a rifle is almost useless as a police weapon if you can't identify your target. Thanks again for yor response.