Decided to build my first AR. I just received my lower Receiver from Brownells and my local FFL. I like to build it for 3GUN competition, target shooting and if possible home defense. I need a little help deciding what caliber, what parts that are reliable and dependable and what parts I should stay clear from? Gas driven or piston driven? What uppers, barrels to look at ect. Any help is appreciated, thanks in advance. .
For get to add, the Lower is from Areo Precision muli cal. Is it normal if you look towards the back bottom behind the grip of the receiver there looks like a defect in the mold or casting?