Just remember that the OP said max $100. On my personal favorite AR I have a Simmons Wally-World 30mm red dot. I bought it years ago when I was very poor, intending on changing it out when I could afford something better. The rifle has been used hard, dropped directly on the scope and its still never had to be sighted in again. I just don't see the need to change it out. Looks just like the one in 4th's pic. Same price range.