Well, what are the considerations for a "trunk gun" (not a truck gun since I don't drive a truck)..I've debated that in my head quite a few times. I've concluded a few things that are purely my opinion.
One of the criteria for a trunk gun would be self defense. In that scenario, a bolt action rifle is simply not acceptable. Semi automatic is best due to rate of fire thus increasing hit probability in a shorter amount of time.
Any rifle meant to be kept in a vehicle must be able to sustain abuse and weather conditions. Vehicles get hot/cold/dry/humid.
Any firearm in a vehicle must be expendable. It must not be a serious financial burden or impact if it is lost or stolen. Thus, the cheaper in cost, the better.
A rifle kept in a trunk would carry out a role to be used when out and about. This could be self defense, this could be used against dangerous animals, it could be used in a SHTF scenario, a riot, to defend against a serious criminal action in public...For that, you need a cartridge that is capable.
A vehicle weapon is likely to be used at either out in a rural area where you'd want serious power, in an urban area, where you'd also want power, or in or around and about other vehicles, where again, you'd want power.
You need a cartridge that will incapacitate even an armored target, such as a lunatic wearing a kevlar vest. You need something that can penetrate medium barriers such as the common walls found in suburban/urban environments. Being a vehicle kept weapon, it must be able to be effective against vehicles. This means the penetration of auto glass, car body and interiors.
The AR platform is too expensive, and aside from using SS109 or M855, not the best penetrator. The AK is also too expensive (to get a decent one). M1 carbine is also expensive, and fires a cartridge that is underpowered for use against structures or vehicles. Although it, like the AK and the AR has the capacity for 30rds.
The best choice is the SKS. It gives you 10 good rounds (or 20 if you get the larger mag) of 7.62x39mm which is ideal at the practical ranges one would encounter for a vehicle kept "out and about" type rifle for virtually any imagineable threat you'd encounter. All at a very low cost, with good reliability, good ability to sustain abuse, and in its original military condition (not converted or butchered up, or a military copy using substandard metallurgy that is junk) It is also tough enough to be used as a blunt force weapon, or if you have a bayonet, well...
It has all the advantages, and no disadvantage. Size/Weight might be one, but this is a trunk gun and not something you carry around or use often.