yup...I'm looking to get an M4 flat top receiver put together for my AR. I bought it with the 20" barrel in stainless. it shoots great, with iron sights I can hit a quarter consistently at 30 yards, so that's what...about a 5" group at 100 yards? good enough for what I use it for.
but, I want shorter and lighter now. I also put the much smaller sights on the upper I have now and it was great, especially for longer range targets, because the further out you reach, the more of the target the sight was covering up. I'll probably get something like that for whatever new upper I buy, along with a red-dot sight, probably in the $150 range. the only other thing I would consider adding would be a flashlight, but probably not. I don't exactly do a lot of night time shooting, and if a bad guy comes knocking at night time, I'm not going for an AR that's gonna shoot through him, and the next 3 walls too. I'll grab the 870, and that already has a pressure switched light mounted on it. great thing with a scatter gun like that, is you can use the flashlight almost like a laser sight. the beam pattern represents your spray pattern! the further away the target, the larger area your flashlight is going to illuminate, AND the larger your spray pattern is going to be.