I received bsa red dot rd30 as gift. What size riser do i need for a M&P15 sport. I know a fair amount about pistols and non ar rifles. I know how to zero iron sights. I know little or nothing about a red dot on an ar. and not much more about ar's (my first)

Art

The riser height is going to depend if you want absolute co-witness, or lower 1/3rd co-witness.

A 1/2 inch riser should get you absolute co-witness and a 3/4" riser will get you 1/3 co-witness. The difference in how they look is this:

I used a BSA RD30 on my 1522 for a week or so. .75 worked well.
Might consider putting the riser money toward a Bushnell TRS 25 or PA micro dot. Under $100.