There's an interesting discussion going on over at the highly-respected Professional Soldiers forum about the best options for carrying a spare mag, particularly around the FOB or COP when you don't have all your gear on.
One poster asked the opinion of the SF crew who populate the forum about whether they thought the Redi-Mag accessory was a good way to go. The Redi-Mag is essentially a manufactured attachment that formalizes what the old-school rebels (think Congo or Angola) have been doing for years taping two or three mags together for rapid reload.
The snake eaters on the blog basically come down to the "go-slick" approach: get a less expensive mag holder for your trouser waist band, a pouch for your thigh rig or, even more down-market, just throw one in your leg or back pocket.
The primary concern with the Redi-Mag is the weight and balance issue the accessory would incur on the M4. They don't go into specifics, but I can see how you'd almost have to over correct with the magazine ounces pulling the action toward the left.
The other point is that the Redi-Mag costs and astonishing $85...a lot of money to spend on an accessory that the experts believe can be fixed with much lower -- or no -- cost at all. One of the posters recognized that these days some folks tend to angle toward the Gucci solution for a half-a-second advantage, while the guys who've been there and done that edge to the KISS method since it's withstood the test of time.
We'd like to hear your opinion on this as well. What do our readers use to carry a spare mag when you're not all kitted up?