Its not an AR15 but its in the AR family
DPMS LR308B
18" Heavy barrel
Seeing that picture brings back awesome memories. I bought one exactly like that and all I did was change the trigger and I was hitting steel plates at 800 yards as fast as I could get on target and pull the trigger. Then we found out baby #1 was on the way and I sold it in preparation of the med bills and expenses to come.
Always missed that rifle and eventually replaced it with a slightly different 308B. Shortly after hitting the steel at 1000 yards with it we got news of baby #2 and I sold it off for the same reasons I did the first one.