Introductory note: This guide is mainly aimed at the American prepper.
Without a doubt, firearms are one of the most important preps we have to make; without a way to defend them, none of the other supplies we amass are truly accounted for. So, when the beginning prepper sets out on the road to self-sufficiency and preparedness, we want to maximize our dollars; buy once, cry once. Perhaps you're not a 'gun guy,' like some of us, or perhaps you're only used to hunting, or sport shooting. Which guns should be considered, and why? Hopefully, this will help.
Gun number one: The Ruger 10/22. .22LR is an incredibly important round to have covered; besides being the most common round in the continent (and world), it can take anything from mice to deer, if it has to, while being one of the most affordable cartridges to store and shoot (current inflated prices and availability, aside.) Additionally, if you are completely new to firearms, starting with a .22 is an absolute must. No recoil, little noise, and lightweight all make these guns easy to shoot. The polymer-stocked version of this gun is preferable for its durability and weather resistance, but not strictly necessary; threaded barrels are great to keep your options open, as well (more on muzzle accessories later).
Recommended accessories: Spare factory 10 or 25 round magazines, a 2-point sling, and an optic (either a red dot or low-magnification scope). The least expensive red dot worth considering is the Bushnell TRS-25 (I have two, on an AR and AK, respectively, and no failures in about three years and 4,500 rounds).
Gun number two: The AR-15. Now, I know many SurvivalBlog readers side with Rawles on the topic of defensive rifles, but I deviate -- and I'll explain why. First and foremost, commonality. The AR is ubiquitous; they are everywhere, in America, and spare parts will be around more for it than any other centerfire weapon. Now, of course, storing your own spare parts is good (and highly recommended), but more options are preferable to less. The AR is also effective to 600 meters (even with iron sights, as many High Power Rifle competitors can attest to; though optics are preferred!), and while this is less than what a good .308 can do, most engagements happen within 200 meters, anyway, and there are other advantages associated with the AR that I feel make up for this slight downfall, for a go-to defensive carbine. The AR has very, very little recoil, both because of the .223 round itself, and the direct impingement and buffer tube system the rifle uses. Without exaggeration, at close range, you can shoot the rifle as quickly as you can pull the trigger, and easily keep all the rounds in an 8" circle, using a red dot optic. This is an important consideration, because statistically, we know that most combat happens at close range, and the AR excels here; the lack of muzzle climb means that you can put more rounds on target more quickly than you can with a .308. We also must have an understanding of what kind of terminal ballistics we want out of our main rifles; instant incapacitation, which is only caused by hitting the central nervous system, the aorta, or the heart. Permanent wound channels and expanding hollow points are neat to read about, but the deciding factor in putting bad guys down is hitting the above targets. Knowing that, being able to put more rounds in them more quickly is preferable in a general-purpose defensive rifle. When buying your rifle, opt for a 16" barrel (M4 contour is usually the least expensive, but when possible, I recommend a heavier barrel; it retains accuracy better when hot). The midlength gas system is preferable, but not necessary; carbine length is almost always less expensive, and will work for you just fine. Always opt for a flat-top railed upper receiver unless you get a really good deal on an AR with an integrated carry handle.
Recommended accessories: Plenty of magazines (Magpul PMAGs, Lancer magazines, NATO aluminum mags with Magpul ant- tilt followers), Magpul ASAP sling plate, Magpul MS3 sling, back up iron sights (Magpul MBUS are less expensive and actually stand up better than metal sights in drop tests), a handstop or vertical foregrip for use with a high-thumb or thumb over bore grip, and a quality tactical flashlight of your choice (the Streamlight Polytac LED is affordable). For an optic, I strongly recommend a red dot sight; Aimpoint is the best if you can afford it, period. The H1 and T1 micros are best for their light weight and small size. For the budgeted prepper, the Primary Arms Micro sights with quick disconnect bases are your best option. For standard rifle and carbine handguards, there are extremely affordable bolt-on rail systems that attach via the upper and lower vent holes. As you can afford it, I also recommend upgrading to a Magpul STR stock; this stock lasts extremely well in drop tests compared to others, provides a better cheek weld, and lets you store spare batteries for your taclights. HSGI TACO rifle pouches are the best mag-carrying option, as well, and I recommend using them with a sturdy rigger's belt for the lightest gear possible; a lightweight chest rig like the Blue Force Gear Ten Speed M4 Rig is good for adding on for maximum carrying capacity.
Absolutely-don'ts: Optic in a non-quick disconnect mount, or internal modifications (they're less rugged than mil spec triggers and parts).
Gun number three: The Glock 19 or 17 pistol. Or, less preferably, a Glock 34. Glocks are the most reliable combat handguns in the world -- period. People who put guns to use when lives are on the line choose them over others by wide margins, and for good reason; they are extremely simple, extremely rugged, and extremely common. Much like the AR, Glocks benefit from having parts and accessory options everywhere. For new shooters, as well, the controls are as easy as can be; unless you have a malfunction, the only parts of the gun used to operate it are the mag release, and racking the slide during reloads. In a handgun, I strongly prefer not to have a manual safety, as well; if a pistol is coming out, it's to save your life, and you don't want anything impeding that. The Glock's trigger safety, as well as drop and hammer block safeties, totally prevent the firearm from discharging unless the trigger is pulled. As for the cartridge; 9mm is preferable to .40 and .45 for several reasons, much in the same style as .223 vs .308. It recoils less, holds more rounds, and is less expensive when prices aren't inflated. Besides that, all three rounds have almost identical wound channels with modern ammunition. A Gen 3 or 4 is what you want to look for; earlier models have compatibility issues.
Recommended accessories: Plenty of factory magazines (with +2 baseplates, if you like), a Surefire X300 or Streamlight TLR-1 weapon light, a kydex light-bearing holster (from Raven Concealment, Statureman, kydexbyparlusk, etc.), and either two- or three-dot sights. Make sure to get some HSGI TACO pistol pouches for carrying magazines, just like with the AR. A threaded barrel is a good option, as well.
Absolutely-don'ts: Grip plugs (they prevent you from pulling out stuck mags, and prevent water and debris from draining out the gun as it was designed), recoil buffers (they prevent a full slide cycle and can cause malfunctions, and also can break apart), and internal modifications or replacements (they're less rugged than factory triggers and parts).
Gun number three: The Remington 870 Express Magnum shotgun. Preferably, one made before 2003; since becoming owned by Cerberus, Remington has had occasional quality control issues. If you're buying the gun in person, inspect later guns; most are fine, but it's something to watch out for. While your AR will be a better defensive weapon than the shotgun, 12 gauge is extremely common, and worth having covered. It will allow you to hunt birds and small game, as well as being a good breaching tool with a shorter barrel. Make sure you get the Magnum version so you can use 3" shells, and not only 2 3/4"; polymer stocks are preferable, but not necessary.
Recommended accessories: Both a 26" or 28" bird barrel, and an 18.5" or 20" barrel for interpersonal use. Make sure you get the shorter barrel with threads to use choke tubes! Patterning is important! Shotguns do not throw walls of death like in the movies, and every pellet you launch is a liability; besides that, you want to destroy what you shoot at, and getting more pellets on target will do that. On the shorter barrel, having rifle or ghost ring sights is important; many companies make aftermarket sights if your barrel came without them. A shell carrier on the stock, and a velcro-based sidesaddle like the ModuLoader are also great for carrying ammunition. A Magpul MS3 sling and a single point sling attachment are good additions, as well. A taclight mounted to the pump is strongly preferable, as your support hand will always be able to manipulate it immediately, unlike mounted that clamp ahead of the gun's slide; you can drill and attach a section of rail to your pump to accommodate this, or attach something like a Magpul MOE forend with an illumination kit. If you want, as well, getting the gun MagnaPorted or Vang-Comped will reduce recoil and improve patterning.
Solvent Trap Adapters: Now, a foreword; if you are comfortable with getting fingerprinted and charged and made to wait for legal suppressors in your state, by all means, get some -- for every gun you have that can accept one. For the everyman, however, many companies are making a handy rainy-day buy. These adapters are made to screw onto various common thread sizes, such as for .22s, the AR, and threaded Glock barrels, and allow an oil filter to be screwed onto the exterior threads. This filter will catch, and allow you to recycle cleaning materials when cleaning weapons -- but it can also be registered and used as a suppressor. With the correct filters, on both AR-15s and AK pattern rifles, the filters do not block the iron sights, either. A quick web search will allow the interest to buy them, and it can easily be done in an extremely discreet manner. They're a good investment, but be warned! Shooting through an unregistered one is incredibly illegal, but having them put away in case you ever need them is perfectly within the law. [JWR Adds: Readers are warned that the legal status of Solvent Trap Adapters may change in coming years. Therefore, I recommend that you minimize ayn paper trail and buy them face to face with cash at gun shows and don't mention your name. And if you must order them from an Internet vendor, then only order them using Postal Money Orders, or better yet, Bitcoins.]
What next? Every physically-capable group member should have an AR and a Glock; doubling up on shotguns isn't as important, nor is doubling up on .22s. I suggest having at least one precision rifle in .308, as well; a Savage 10 FP-SR is an incredibly good value. And, I must admit, getting a PTR-91 and a backpack full of $2 mags when bought in bulk is not a bad idea. Besides that, make sure you have plenty of ammo safely stored. I would consider 3,000 rounds per fighting rifle a minimum! More is always better, and make sure you practice. For carbine and handgun, after you have basic marksmanship down (I suggest an Appleseed shoot; check their web site for meets near you), I strongly suggest looking at Travis Haley's Adaptive Carbine, and Adaptive Handgun. Many quality instructors have free videos available on youtube, or elsewhere.
Introductory note: This guide is mainly aimed at the American prepper.