I second what Zardoz has to say. [:)]
I own a Bushmaster M4A3, and I love it! I've put Hogue grips on it (today) and M33 handguards for a vertical foregrip and/or a light, and the flat-top will house my ML2 (once I get the #22M68 mount and spacer!). It was my first AR, and I would have been fine with just that one, had it not been for the fact that once you own one AR, they are like rabbits: they breed more of them! I had to get a 20" flat-top kit (had to build it myself, too!), and now, I'm likely getting yet ANOTHER M4. Why? I don't really know, but the voice in my head says I should build another one.
The voice? Oh yeah, it's the voice of the Mighty AR, God of Guns. You'll meet him soon enough.
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It's really late. Perhaps I need some sleep...