Omnibus Crime Control Act of 1968
Usually referred to as the Gun Control Act of 68 (GCA 68). Fed law mandating licensing of gun dealers, setting age limits on gun purchases, filling out forms, and a whole bunch of other stuff as well.
Texas may have added their own state forms and requirements afterwards, I don't know, sorry. Some states do, others don't.
All else being equal (and it almost never is) bigger bullets tend to work better.