As far as the Assault Rifle/Rifle/Shotgun issue, here’s my take;
Rifle; with a good scope and rest (bi-pod, beanbag, forked branch, etc…), headshots are possible at extreme distance, e.g. the .22LR cartridge has a range of 1.2 miles, barring wind influence, ballistics and bullet drop, admittedly, the energy deposit at 1.2 miles would be negligible, but the round is viable to at least 300 yards, larger calibers will have greater chances of skull penetration and energy deposit, but at the cost of increased weight
Single shot break-barrel rifles are the quietest (no breech for sound to exit from) and generally the most accurate, bolt actions are just as accurate, and can be had with a clip-fed or tube-fed design, holding up to 10 rounds, semiautos are a hair less accurate, but more convenient, and will fire as rapidly as you can pull and release the trigger, clip fed and tube fed semis are the norm here, and guns like the Ruger 10/22 can take large capacity magazines as well (10-50 rounders), and can also be heavily customized with different stocks, scopes, and barrels
An excellent Zombie-slayer 10/22 (or any other rifle) setup would be;
polymer stock with a bipod
zoom scope
laser sight and weaponlight
“Bull” barrel (heavy barrel, helps to steady the weapon when shooting)
Suppressor (the .22LR is quiet enough already, a supressed .22 barely makes any noise at all, less noise to attract Zack)
long barrels are unneccecary on a .22LR, as the bullet has achieved maximum speed 18" down the barrel, longer barrels stabilize the bullet better (more rifling), but also slow the bullet down a bit (frictional drag after 18")
using the Zombie Survival Guide as a, well, guide…, Brooks reccomends the .22 Long Rifle (other variants are the more powerful and longer .22 Magnum, and the .223 Centerfire (5.56 NATO))due to the fact that a large amount of ammunition can be carried with a minimal amount of weight, the drawback to the .22LR is the small, lightweight projectile, you’d better be aiming to hit an eye socket, the temple, or the nasal cavity, the .22 may not have the neccecary power to crack the human skull
Assault Rifle; larger, more powerful cartridge, long range, but too much temptation to switch to a 3 round burst or full auto and “rock and rolling”, too much indiscriminate lead-throwing, wasting perfectly good ammunition, a single headshot will drop Zack in his place, Rockin’ and Rollin’ may be fun, but it’s a waste of valuable ammunition
Shotgun; the general “belief” about shotguns is that “you don’t have to aim”, but at the typical Zombie-killin’ range, the shot cloud has not had much time to expand, so you really DO have to aim, if you encounter a Z that’s within lets say, 10-15 feet of you, you could theoretically miss it, shotguns (lets say a 12-gauge, as that’s the most common and most popular caliber) are also heavier than rifles, and have a heavy kick, which can fatigue the shooter during extended shooting sessions, if you are using slugs in a shotgun (think BIG bullets), then you’d have to aim just like a rifle anyway
shotguns also create a huge mess on close-in Zack shots, a headshot from a 12-gauge using #4 Buckshot or bigger would create an aerosolized mist of Zombie-head, and that fine red mist of Z Cranial Contents would create a possible infection vector if the shooter was to breathe any of it in, wheras a rifle would allow you to drop Zacks from a distance
Handgun; inaccurate beyond 20 feet under stress, especially when trying to hit a small mobile target like a Zombie skull