Quoted:
Vertical vs horizontal is a much more natural position for your hand and forearm. That being said, I need to get one myself!
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Yes your hand & wrist is in a more neutral position - but its a very poor position for bearing the weight and you will have to use more muscles to hold up the front end.
The *propely positioned* foward grip will allow better muzzle control. It also puts the wrist in a more neutral position reducing fatigue in the support arm muscles. With the support elbow tucked in you get much better mechanical support for the weight (some can be transferred to the body). All this results in less fatigue when clearing buildings (which can take a while).
At first I didn't like the grips. It turned out I had tried them too far away from my body. After reading some tips from Pat Rogers in one of his articles I tried it again (this time closer to the magazine well) and it quickly grew on me.
One of the cheapest forward grips I've found is the one sold by Fulton Armory (VG-1 Guardian Foward Grip). It normally costs $24.95 and right now its 20% off (till Christmas) so your cost if you order now is [b]$20[/b] plus shipping.
The grip itself is a copy of the Knight's grip. Its all plastic, and tightens via a knob at the bottom which raises a piston with a 'stub' to lock into the rails. This grip works well with the Fobus M33 & M44 rails (I've been it on my carbine). The grip is a bit thinner than the Knights, but costs less than half.