These are by far my favorite lowers to use for a build. Upper tension screw, threaded bolt catch pin, flared magwell. $50 at Brownells.
https://www.brownells.com/gun-parts...s-parts/ar-15-billet-lower-receiver-c356187f?
https://www.brownells.com/gun-parts...s-parts/ar-15-billet-lower-receiver-c356187f?