Just an FYI for anyone wanting to do this:

I purchased a Magpul K2 XL grip.  

1. The hole in the receiver isn't the same thread pitch as an AR so the Magpul screw that comes with it won't work.
2. The Barrett screw from the factory grip is also too long to hold the Magpul grip before it bottoms out.  It was too wobbly and after a little bit I figured out that I needed to shim up the screw.  I ended up using a nut in my spare parts box which happened to be the correct threading and that shimmed it up enough to get a tight fit on the grip.

I got a shorter bolt from Lowes. I'm pretty sure it was less than $1.50. A slight inconvenience, but a simple enough fix.