it's going to depend a lot on what you need it for.
I lost a nice AR to Theft from my truck , so I don't leave guns in truck anymore
I also have the issue of teen drivers now , who borrow my truck some times , they can't go the same places I can with a gun in the vehicle.
now I have some guns that are easy to grab and take with but come back in the house when I come home.
of those the one I would grab first is a Mossberg Patriot Predator in 308 2-7x32 scope and a Hornady cheek riser bag with storage that holds 2 loaded mags and 5 rounds in loops it also has a muzzle brake on it makes it light recoiling and protects the crown. flip up caps on the scope.
it can sit next to the door and be grabbed and it is ready to go with 15 rounds of should be able to kill anything in north america 180 gr soft points that it is sighted in for and I know where to hold to 400 yards
there are a couple different major schools of thought on truck guns
A. cheap this is it will get beat up , cheap tools can be ok but what are you giving up in performance , durability , reliability to save on cost.
B. technically a pistol. In WI you have to unload a long gun although we used to have to unload them and have them cased and out of reach. With a WI CCL they only have to be unloaded. Handguns/ Pistols can be loaded (having a mag in even with no round in the chamber is still loaded technically)
C a good tool for whatever your intended target is , adding that a synthetic stock , lock-tite everything you don't want moving , a good optic if you run one and I think a muzzle device is a good idea chances it will end up muzzle down on the floor of the truck between the front seats while you drive around is real something protecting the muzzle is a good idea in my opinion. yes your going to have real money into a real good tool but if you are saving cattle that bring good money , having good tools that work make sense when it is something you need to get done.
this is a pistol it has a 10.5 inch barrel it is chambered in 5.56/223 it has a prism sight it works red / green and also with a dead battery it is black it has an etch glass reticle that can be lit either color or if daylight no batter power is really needed.
it shoots decently well at 200 yards well enough to keep on the steel plate which while running that optic on a 16 inch barrel AR was really all the better I could see. short barrel means a little less velocity but no less accuracy.
it could fall into B and C you loose about 10% of the velocity from a 20 inch barrel if you go shorter it starts to drop off fast and about 8% compared to a 16 inch barrel
if a 17hmr inside 100 yards was going to get it done , a 223 even 10% reduced power should definitely get it done to 250 or better.
it is also fairly light 6 pounds even and easy to carry on a sling across the back , doesn't take up much space
whats a good tool for your purpose? then what do you do to make it ride around in a truck better?