I have researched this in the past, specifically for Virginia, but not other states. I personally do like the idea of having a backpack gun (generally an AR) and/or truck gun that goes with you in a backpack. Obviously, other things have to be taken into consideration as well in terms of if you would ever have to leave that backpack in your vehicle or if it could always be with you. Below is the statute specific to VA, I am most definitely not a lawyer but from my understanding of what it says at the end “anyone having a valid concealed handgun permit” is exempt from the law that otherwise wouldn’t allow this.
" § 18.2-287.4. Carrying loaded firearms in public areas prohibited; penalty.
It shall be unlawful for any person to carry a loaded (a) semi-automatic center-fire rifle or pistol that expels single or multiple projectiles by action of an explosion of a combustible material and is equipped at the time of the offense with a magazine that will hold more than 20 rounds of ammunition or designed by the manufacturer to accommodate a silencer or equipped with a folding stock or (b) shotgun with a magazine that will hold more than seven rounds of the longest ammunition for which it is chambered on or about his person on any public street, road, alley, sidewalk, public right-of-way, or in any public park or any other place of whatever nature that is open to the public in the Cities of Alexandria, Chesapeake, Fairfax, Falls Church, Newport News, Norfolk, Richmond, or Virginia Beach or in the Counties of Arlington, Fairfax, Henrico, Loudoun, or Prince William.
The provisions of this section shall not apply to law-enforcement officers, licensed security guards, military personnel in the performance of their lawful duties, or any person having a valid concealed handgun permit or to any person actually engaged in lawful hunting or lawful recreational shooting activities at an established shooting range or shooting contest. Any person violating the provisions of this section shall be guilty of a Class 1 misdemeanor.
The exemptions set forth in §§ 18.2-308 and 18.2-308.016 shall apply, mutatis mutandis, to the provisions of this section.
1991, c. 570; 1992, c. 790; 2003, c. 976; 2004, c. 995; 2005, c. 160; 2007, c. 813; 2016, c. 257."
This is one of those things that is going to be different depending on the specific state.