I had to look this up due to my concern of being on the mexican border in light of recent issues of homicides due to drug traffic lately. I'm no longer concerned. The AR and a pistol will be in the XK with the family.

FYI:
"As of February 22, 2010, a new federal law allows people who can legally possess firearms under applicable federal, state, and local laws, to legally possess firearms in this park. It is the responsibility of visitors to understand and comply with all applicable state, local, and federal firearms laws before entering this park. As a starting point, please review this PDF of Texas Firearm Regulations

Federal law also prohibits firearms in certain facilities in this park; those places are marked with signs at all public entrances.

Keep in mind that the regulations to carry a firearm do not permit the discharging of a firearm. The discharge of a firearm is still illegal in national parks."