The only AR-15 I know of which has a built in brass catcher is an AR-15 lower with an AR-57 upper in 5.7x28 FN. With this setup the modified 30 round magazine in the AR-15 lower serves as the brass catcher and will hold all 50 shots of the FN brass. The brass catcher releases like a normal mag and dumps easily. The mod is easy, just remove the spring and follower and cut the magazine off flush with the top of the lower receiver, a single straight cut. The 5.7x28 isn't an especially "long range" cartridge but it's good for varmints to 200+ yards. This isn't mine but I have a pistol version which works the same way. Without the modified mag in the lower it just drops the brass on the ground.