|What to do?|
Reloading is the obvious answer, but if you're at the point where you and your personal treasure trove of ammo have been parted, there's a good chance you're low on reloading components--namely primers, powders and bullets. So we've got to look to unconventional methods - scavenging components from other caliber cartridges and making your own components by casting lead and mixing up black powder.
Revolvers, pump/single/double barrel shotguns, bolt and lever guns stand out here - less strict requirements on powder loading, bullet types and so on than a semiauto. An AR-15 is probably not going to run very well with cast lead bullets and black powder.
Of course, mess around with any of these at your own risk. Firearms, fingers and eyeballs are all things you don't want to blow up.
One scene from the book version of The Road has always stuck with me - where the dad finds a couple boxes of ammunition and wonders if he can somehow use 'em to load his .357 magnum revolver, for which he has only a round or two at the time. It's been something I've wondered about.
Scavenging the components from ammo isn't terribly hard--reloaders disassemble ammuntion all the time. An impact bullet puller is made for this purpose - a couple whacks and the bullet comes out, along with the powder. A vice and pilers can accomplish the same thing with a bit more trouble. With care, you can remove and reuse an unused primer, too - normal primer removal methods will work.
If you're going to be able to use scavenged components, you'd need to know what primers you needed and what sizes of bullets would work.
There are 2 standard primers sizes for pistols and rifles--small pistol & large pistol, small & large rifle. There are also magnum versions of each, paired up with different powder loadings that require a hotter/faster primer to burn correctly. As an example, .25 ACP, .32 ACP, .380 ACP, 9mm Luger, .38 spl, some .357 magnum loads and many others use small pistol primers. If you had a box of say 9mm and but only had a .38 revolver to shoot 'em out of, you could certainly put the primers to use.
Bullet sizes get a bit trickier - you've got bullet diameter, length and weight to worry about--diameter is the most critical to worry about. Luckily, many calibers use the exact same diameter bullets--for example, .30 Carbine, .30-30, .308 Win, .30-06 and .300 Mag all use .308" diameter bullets, while 9mm Luger, .380 ACP and .357 Sig all use .355" diameter bullets...and .38 spl and .357 mag use a very close .357" bullet.
Here are lists of pistol (pretty exhaustive) and rifle bullet diameters for your perusal. Bullet diameters can and will vary by small amounts - thousands of an inch - and fall within spec.
Shotguns have an advantage for scavenging - you can open up a shell with a pocket knife, they use the same primer size (209 - though there are magnum and other speeds of 209) and you can cram
Scavenging powders would be tricky business unless you knew the exact powder that you were dealing with. You probably wouldn't, as manufacturers don't typically use the same stuff you buy off the shelf and they don't plaster it all over the box. So you'd be using an unknown powder to switch between loads. A reloading manual and knowledge of powder weights, SAAMI pressure specifications and so on would be a starting place to avoid blowing your gun up with an overcharge of mystery powder. For example, 9mm Luger typically uses less powder and runs at similar pressures of a .357 magnum, so you would be less likely to blow up a .357 magnum with a powder charge from a 9mm.
You would, of course, in all cases, start out with as light and safe of loads as possible.
Rolling Your Own Components
There's lots of information out there on casting bullets - Google up the topic - basically melt lead, pour into moulds, trim as necessary and lube. For shotgun shells, there are lots of viable improvised loads - Dave Canterbury and others plenty of have videos up on YouTube. Not too difficult, and lead isn't too difficult to source.
Black powder could be used in some weapons - doubtful for semi autos, but it's certainly viable in things like revolvers, pumps and lever guns. Many cartridges that we have today started out as black powder loads - the .45-70, .45 Colt, .38 special, various shotgun gauges and so on. I'm not going to venture to walk through the process of making black powder today, but one of the better books on the subject of making your own black powder is the Do-It-Yourself Gun Powder Cookbook from our friends at Paladin Press. Sulfur is the trickiest of the three main components to source, so your ability to cook up gun powder in a post-collapse scenario would vary.
Dave Canterbury has several video up on loading 12 gauge with black powder, and loads can be found in reloading manuals and on the interwebs.
Primers are tough. According to an old CIA insurgency manual, you can reload spent primers with smashed up heads from strike anywhere matches. From accounts, this will work...but you'd need a source of strike anywhere matches. Beyond that, I'm not sure what else would work - and without primers, you're pretty much out of the modern firearm discussion.
Reloading ammo with scavenged or home made components is not the simplest and safest of tasks, which is one of the many reasons why it's smart to have a stock pile of ammo, and probably some redundant caches of the stuff, too. It also pays to have firearms in standard calibers, especially police and military calibers (anyone know how much 5.56 the military has squirreled away?). But, having some of that knowledge up your sleeve, just in case, never hurts either. Something to look into!