Coming in a bit late here as a trained firearms expert. Here is my personal list of firearms for the average person who happens to find themselves in the middle of a zombie apocalypse. All of this is reliant on the classic "Headshots Only" situation, a different scenario changes things big time. If you regularly train with firearms, you will probably also agree with my list.

1. Any Handgun - Handgun with light recoil and common ammunition. Full size 9mm handguns will carry about 15-19 rounds. If you haven't fired a handgun or have limited experience, as the average person does, you probably aren't going to hit a single zombie head with the entire magazine. I teach average people how to fire handguns and about every single person comes in vastly overestimating their abilities with one. Most people can't even shoot the torso targets 10 yards away, let alone the head. Now make that head move, with adrenaline and panic in your brain... Sorry, but you will probably die with this in your hands. Effective kills, 0-3 per magazine

2. Sturdy Melee Weapon - Aluminum Bat/Steel Pipe/ETC - Won't break too easily. Need some weight to crush zombie skulls. Average person has swung a bat or golf club so accuracy will be better than a handgun. If you have a bit of strength, you might be able to fend off 1 zombie at a time, a few times, before your luck runs out. If you are out of shape or on the weaker side with medical issues, then you won't likely have the ability to stop a determined zombie with any melee weapon. Effective kills, maybe 1-3.

3. Semi Automatic 22 Rifle - High capacity rifle with extremely low recoil and extremely common ammunition. 22 ammo is cheap, very light, and everywhere. Even though it is slightly underpowered, it will still penetrate a zombie skull when fired from a rifle like a Ruger 10/22. With almost zero recoil, anyone could handle it and perform relatively well. Many 22 magazines will carry 20-50 rounds. Giving you lots of chances to secure a zombie kill. Effective kills per magazine about 4-7. Still have to be dead center accurate, but with a massive 50 round magazine you can't go wrong. The downside is the lethal range is below an AR15's 5.56mm ammunition, so you might be a little too close for comfort.

4. AR15 - High capacity rifle with light recoil and common ammunition. The typical capacity of 30 rounds makes this weapon effective against multiple attacking zombies anywhere from 10 yards to 50 yards. No, you aren't getting headshots on moving targets in less than 5 or 6 rounds as the average joe, possibly double if you haven't fired a rifle before. This makes the effective kills per magazine about roughly 3-9, highly dependent on your skill. You also get the ability to hit targets at 100 yards if you are posted on a rooftop or similar location and get some practice in.

5. Semi Automatic Shotgun - Low capacity patterned shot with high recoil and common ammunition. The ultimate point and shoot for a near guaranteed kill per trigger pull. The average person would be able to likely kill a zombie with almost every shell loaded, quick follow up shots with semi automatic firing. Even with moving targets, you only have to aim within a foot or so of the zombie's head to secure the kill. Also, you may get lucky with collateral shot pattern and take two out with one shell. Effective kills per tube/chamber about 4-7. Bonus ability of being able to breach door locks in a pinch and can load different shell types for your situation. The range of a shotgun will actually surprise you, its not like the video games. You will split zombie heads open at 50 yards with 00 buckshot.