I got a KAC vertical grip off the EE. First time I've had one. Please tell me it is supposed to have some sort of crossbolt that secures it to the rail that I'm missing. It looks like it has holes for it. All there is right now is the plastic nipple on the top of the screw in center core to keep it from sliding forward/back on the rail. That doesn't seem strong enough at all.

Where can I get just the part I need without buying the whole thing again?

No it has no crossbolt.  The hole is to allow you to see that the nub (on the top of the piston) will fit in the rail slot.

The handle is held in place by the piston that rises when you turn the bottom.  There is a nub on the top of the piston to help prevent movement - and to insure consistant placement on the rail.

Quoted: The handle is held in place by the piston that rises when you turn the bottom.  There is a nub on the top of the piston to help prevent movement

Thaks for the reply Forest.

Yep. that's what I was referring to as that plastic nipple on top of the screw in piece. It sure looks like a weak point but I haven't read about people complaining about it breaking off so should I assume it's really not a concern?

Quoted: ....so should I assume it's really not a concern?

Correct.  They have been used by the military for years (and still are).

There is one company that makes a clone of the KAC that doesn't even have the nub/pin - yet it still works (I had one of these for a while).

If you want TOUGH though get a Tango Down VFG.

In true AR15.com fastion I have one of each - KAC and TD.  The lighter (and better feeling IMHO) KAC rides on my carbine; while the tougher TD is on my rifle.

If you don't align it properly during tightening, you will damage the nub.  Also, you need to tighten the crap out of it to keep it from slipping during use.  If it slips, it will shear the nub off.