Okay so I'm fairly new to ar's. I just bought a s&w m&p 15 sport and I want a red dot and a flash light for it.  I want to keep them combined under 250 dollars... I don't know much about optics so what are good red dots for under 150?  I prefer open rather than a tube but if a tube stands out than I'll get it.  Also how can I be sure the red dot will fit on my quad rail?  Are they universal?  My dad bought me a Simmons red dot that didn't fit and I don't want to make that mistake again!  Another thing is I like the flashlight mounts that mount right under where the front iron sight is.  What good lights would fit in that?  You don't have to answer all my questions and I apologize for asking so many but there all so many brands out there I thought I would ask people who know.  Thanks.

Bushnell TRS-25 Hirise red dot
Inforce WML light.

Done! Enjoy.

Both of my local walmarts have the Bushnell TRS/rise combos in stock for $109.

Also check out Primary Arms for cheap red dots.

Another vote for the Bushnell TRS-25 either with UTG medium riser ($10 at midwayusa), absolute cowitness or the TRS-25 hirise with mount lower third cowitness.

OR

Primary Arms micro with mount. PA has several options.

I have a TRS on a AMD-65 that's been dropped and thrown around with no loss of zero. +1 to trs25.

I always used a streamlight polytac with a cheaper flashlight mount on my rifles.
So far a lot of good info to go with. Just keep both eyes open when shooting the TRS-25 and it's like a dot floating in mid  air