The Remington gun is actually my main gun
Well there's your problem...
'Cept I'm not really having a problem, I'm just figuring stuff out about guns.
Shotguns are too noisy to use as a main gun; even a single shot will alert every zombie within a very large radius to your location.
And I have enough ammo to kill every zombie that comes along, and when I run out, I have more guns, and my car is nearby.
Sorry mate, I'm not yet at the point of "Paranoid Veteran", if I'm not shooting absolutely everything at all times then I'm probably running them all over with a car, so what if I only have a 30% survival rate past day 5, I have fun doing it, and past that point it becomes a bit easier to survive for a while until I die to Robots and stuff in a lab somewhere.
Slug ammo behaves differently from shot. A slug is one big bullet, while shot is many small pellets. I'm not sure if shot can crit at all, which would explain the disparity.
I wasn't aware of any difference between the two existing in-game besides the stats, at any rate, 00 shot works for critting just as well as the slugs so that isn't the case.
Personally, I would leave the 870 home to be used later...
Home is where the heart is, and keeping it in my trenchcoat works for that.
No offence, but I didn't ask for advice as to what kind guns I should take into battle, I'm pretty well set into my play-style regarding guns and that is using whatever gun I very well feel like, even if taking something smaller would be more practical.