I've shot a lot with and without, with M-4's and M-16A2's. For longer range (300 meter qualification) and accuracy (25m zero) I always took my forward hand grip off. I never used one at all without a rail mount. The biggest advantage of them is keeping yourself from frying your hand. We had Knight's Armament rails on our M-4's in the 82nd Airborne Division, and leading up to deployment we did a lot of CQB live fire training, firing enough rounds in ready-up and similar drills that you could blister your trigger finger if you didn't wear good gloves. In that kind of firing, the rails would get hot enough to burn your hand through the glove, hence everyone used a forward hand grip. Overseas, we used a 150 meter zero rather than a 300, and everyone left their forward hand grip on their rifle.. Some of us had a "normal" one, a few had a bipod version (it makes for a wobbly bipod though), I used a version with a built-in Surefire light, which was very handy since I didn't have to have an extra piece hanging on the rail, and I didn't have to worry about snagging a taped-on pressure switch and wire, plus it weighed a little less than a separate Surefire and hand grip. When you carry your rifle on patrol nearly every day for 18 months, the littlest things like that are a luxury..
