No, no, no! You must shoot your AR single shot, don't buy any evil magazines. If you do, you stand a good chance of contracting the disease "magaholicism." This disease forms when one tries to find the answer to the age ol' question, how many magazines are enough form my ________? (you may have already contracted this disease or be especially suseptable to it through your association with your 1911) It is especially dangerous when experimenting with ARs as there are so many good manufacturers of USGI mags and it is impossible to come to a certain conclusion as to which one is the best. (Aside form the Coopers mentioned and a few quirky Sanchez mags) One minute you'll be happy with your two 20 rd Adventureline mags, then you'll have to see what all the fuss is about the Colts. Oh, but what about the 30 rounders? And was that really a Sterling 40 rd mag I saw at the gunshow last week. Before you know it, it's too late! So stick to 1 round at a time...
Really, USGI are the way to go with thermolds second and orlites third. Though, nothing looks better in a photo than a thermold.
Under no circumstances should you buy cheap steel magazines - those manufactured by USA, Triple K, Western, etc. They're all duds. You generally get what you pay with mags.