One of my AR's is a Bushmaster XM15-E2S. The gun shoots great, but it started to feel a bit "loose" after the first 500 rounds. A friend of mine recommended buying and inserting an Ar15/M16 Accu-Wedge (Bushmaster part # ACU1B) in my lower receiver. I put one in and the gun feels a lot more solid when I shoot it. Just a suggestion.