We have not found any Texas laws that restrict transporting a rifle or other long guns in a motor vehicle, but Texas law does address the legality of carrying a handgun in a motor vehicle. Texas Penal Code Section 46.02(a)(2)(b) provides an exception to the unlawful carrying of weapons offense that allows carrying a handgun under certain conditions:
Sec. 46.02. UNLAWFUL CARRYING WEAPONS. (a) A person commits an offense if the person:
(1) intentionally, knowingly, or recklessly carries on or about his or her person a handgun; and
(2) is not:
(A) on the person's own premises or premises under the person's control; or
(B) inside of or directly en route to a motor vehicle or watercraft that is owned by the person or under the person's control.
Additionally, section (a-1) states that only a person with a Texas License to Carry is permitted to keep a firearm in “plain view” within the vehicle:
(a-1) A person commits an offense if the person intentionally, knowingly, or recklessly carries on or about his or her person a handgun in a motor vehicle or watercraft that is owned by the person or under the person's control at any time in which:
(1) the handgun is in plain view, unless the person is licensed to carry a handgun under Subchapter H, Chapter 411, Government Code, and the handgun is carried in a shoulder or belt holster; or
It is important to keep in mind that other laws may prevent one from carrying a firearm at any time. For example, see section 46.04 of the Penal Code, Unlawful Possession of Firearm.
The Giffords Law Center, a nonprofit devoted to preventing gun violence, has a helpful explanation on their Guns in Vehicles in Texas page if you would like a more in-depth explanation of the Texas laws regarding guns in vehicles. The library also has more information on Texas and federal laws that govern where a firearm can and cannot be carried on the Carry of Firearms in Texas page of our Gun Laws research guide.