I agree with AT, the C15 would be an insane choice for a duty rifle. Definitely look at his recommendations, down the road you'll be glad you did.
BTW, what does your agency allow and/or recommend? See what they say.
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