Hi all.  I just completed ordering 99% of my parts for my first build.  I want to get this angled front grip:

http://www.fab-defense.com/en/category-foregrips/id-699/ergonomic-pointing-grip.html

I have a BCM Keymod handguard on my barrel.  My question is, the PTK is 130mm, which is ~5.1 inches.  From the picture in the link above, it appears that the grip simply slides on to the rail (I have no idea).  My question is, how long of a keymod rail section should I buy to support this?

Joe Bob Outfitters has a 6" section for $28:  http://www.joeboboutfitters.com/Seekins_KeyMod_Rail_Section_13_Slot_6inch_p/skn-keymodrail-13slot.htm  but I don't know if I should try to keep it as short as necessary or if that's important.  I've seen other 5.5 inch rails and was wondering if even a 4" rail section would work, but I'm not sure.

Can anybody tell me what length rail section I should be considering to mount this?

Well, I hope I'm right, but I noticed in the picture that there was one bolt to tighten it, right in the middle of the grip, so I went with the 9 slot, which should be right around 5 inches and plenty to fit the length of the grip.  I also got the FAB Defense VTS for thumb support on the side rail, and purchased a 4 slot keymod rail to mount that.

We'll see how it works out.  Still interested to see if anybody has any other opinions.