Vertical fore-grips are one of those accessories that folks either really like or really dislike. There's little middle ground between the side that prefers either no grip at all or an angled grip at best and the faction that prefers the vertical grip.
For the latter, the Wilcox Stubby Steady Grip is a solid option. Constructed of aerospace-grade aluminum and weighing in slightly less than 4 ounces, it is a rugged yet lightweight option for steadying your carbine.
And, as a co-worker put it, should you need to use it in such a manner, you can hammer nails with it. Durability and stability are its strong points.