A backpack gun is an excellent way to keep a handy firearm at the ready for situations where it’s not convenient or practical to carry a full rifle. You’ll find a lot of companies making bags specifically designed to hold an AR-15 pistol or rifle, and there are also some great options in existing backpacks from other brands.
I’m a little lukewarm on the idea of trunk guns and am definitely not a fan of rifles being kept in a backpack, but I understand that some people do want to have a handy, legally stowed, unloaded AR-15 on them at all times for self defense purposes or other reasons. It’s a good alternative to carrying a full-sized rifle in your car, and it can be a lot more discreet than bringing out an actual gun case and setting it up on the front seat.
The Vertx Ruckpack is one of the better backpack gun options I’ve seen in 2021, and it also happens to be a decent price. It’s big enough to easily accommodate a Glock with a pistol brace or a Sig P320/P250, and it has plenty of other pockets for all the other gear you normally bring along.
The other nice thing about this pack is that it comes with a folding buffer tube adapter, which negates the need to disassemble an AR-15 pistol or rifle to fit it in most backpacks. This means you can just pull the gun out of the bag, flip the buffer tube over, and deploy it in seconds.