Cygnus is dead-on with his post. In simpler terms: Simply having the same last name as someone else does not necessarily entitle that person to adopt and use the other person's arms. In most cases it must be proven that there is a direct line of descent from the original armiger to the potential armiger before it would be appropriate for a descendant bearing the same surname to use those same arms as his or her own. Without showing any evidence of descent from the original armiger, or obtaining their explicit permission, it would be dishonest, disrespectful, and misleading for someone to use the original person’s arms as their own. Instead, the person who wants arms of his or her own should design new arms to represent himself and his descendants. There are many organizations that Americans can use to assist them in this process.