There are a whole slew of forward grips for AR-15 / M-4 rifles. Some fill the role of a hand stop and others are true vertical grips. The Bravo Company Gunfighter Vertical Grip – Short, bridges the gap between the two.
It is very easy to run this as either a hand stop or a vertical grip on your rifle. In addition to that, you can store batteries or other small items inside the waterproof grip and it is easy to put on or remove without needing special tools.
You can find the BCM Gunfighter Short Vertical Grip here.