Now I guess we just slap the upper on the lower and off to the range? Not so fast padawan.

Does the upper fit the lower? Loose or tight can be acceptable. Too tight or no fit and you might have a problem with tolerance stacking. Here is a term that is sure to start a discussion if not an out pissing match over why your upper receiver won’t fit your lower receiver. Both items are mil spec but mil spec is a range and one part might be at the edge of one range and the other part is at the other range its mil spec, they might not fit at all. Your options? Try another upper and lower receiver. If you cannot do that then get out your good file and reamer and start taking off some of the black anodizing (or cerakote). If the fit seems really loose then add one of the those orange Accu-Wedge plastic wedges and tighten that fit right up. AR15 rifles can have a perfect fit but most just fit well enough and even allow a little light between the parts. You are not building a piano (unless you’re paying $2,000 plus, don’t expect perfection)

Have you checked the operation of the lower? Make sure the fire control group works as designed. With an empty mag does the bolt lock back? Does the disconnector work as designed? Does the selector work as designed?

Complete function complete? Grab some good ammo and head off to the range. By good ammo we mean factory fresh, new in the box ammo. Don’t run your new gun on plinkers or seconds or reloads. There will be plenty of time for that later after the gun is broken in (500 rounds or so). Shoot the type and weight of bullet that you plan on mostly shooting to make sure it feeds and cycles correctly.

Now you want to put something better than iron sights? That’s a topic for another day.

Enjoy your new AR-15!

Remember, All SAA Complete Uppers carry our exclusive Forever Warranty. They are all head spaced and test fired.