I'd find out what local LE uses in your area (brands and configuration). There's a greater chance they'll be using S&W (not the Sport though) over the new kid Windham.
Get those specs and check back, we can help explain them if you want.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek