On an AR style weapon there is no cast built into the gun,so it doesn't matter which hand you shoot.
I own several,and I shoot lefty,and have never had an issue with brass coming out the ejection ports or operating the controls.
On shotguns,get a Browning BPS or an Ithaca 37 20ga pump. Both are bottom load/eject,so either are left handed or right handed.
Remington used to make the 870 & 1100 in left handed 20ga-find a used one.