In our discussion about the percussion lock previously, we'd mentioned that the inventor (the Rev. Alexander Forsyth) had used mercury fulminates to set off the main charge. Mercury fulminates continued to be used in priming caps for early centerfire cartridges as well, into the end of the 19th century. However, when people started to switch to using smokeless powders, they began to discover the downsides of mercury fulminate. One of the issues was that mercury fulminate tended to degrade when kept in storage. This was not really an issue when using black powder cartridges, because black powder ignites a lot easier than smokeless powders. However, once people started to switch to smokeless powders for extra power, they found that keeping the cartridges in storage would cause the mercury fulminate primers to degrade so much that they could not reliably ignite the smokeless powder, causing misfires and hang fires. One more problem with mercury fulminates was that in conjunction with smokeless powders, it tended to form copper and zinc amalgams in the brass cases of cartridges, thereby making them unsuitable for reloading.
Due to this, the US Army switched to using potassium chlorate primers in 1898. Some other manufacturers used sodium chlorate instead. While these primers did not degrade as much as mercury fulminate, there were some other problems that came with them. When fired, these primers would decompose and leave a residue of potassium chloride (or sodium chloride) behind in the barrel. Those of you who remember your chemistry lessons in school might remember that sodium chloride is the scientific name for common salt. Potassium chloride is also another corrosive salt. These salts are highly hygroscopic (i.e.) they tend to attract water, especially when in humid conditions. Guess what happens when you have salt and water applied to an iron or steel surface -- that's right, it rusts. Therefore, if the barrel and action are not cleaned after firing such cartridges, there's a good chance that they could rust soon after. Swabbing the surfaces with oil will not prevent these salts from attracting water and rusting the metal.
As a result of this, primers using non-corrosive chemicals were developed in the 1920s, but these were generally used in civilian ammunition only, as the early non-corrosive primers did not last as well in storage as corrosive primers. Due to this, military ammunition tended to use corrosive primers and this was indeed the case for US military ammunition until the 1950s or so. Some other countries (e.g. former Soviet Union, China, Yugoslavia, Bulgaria etc.) continued to use corrosive primers in their cartridges for much longer than this, well into the 1970s and 1980s. Therefore, depending on the source and the age of the ammunition, the user must be wary lest the ammunition is corrosive.
So how does a user ensure that his firearm doesn't rust after using corrosive ammunition. The good news is that this is fairly easy to handle. It turns out that these corrosive salts dissolve in water. Therefore, cleaning the firearm thoroughly using water or a water based lubricant should do the trick. Some people use hot soapy water, others use plain water, still others swear by windex glass cleaner (which is largely water based). After washing off the residue, the firearm should then be dried, then cleaned with normal bore cleaning solution and oiled, as per the normal cleaning procedures.
The firearm should be thoroughly cleaned as soon as possible after firing the corrosive ammunition, to ensure that the corrosive chemicals are removed before they can damage the firearm. While this may seem like a bit of extra work, it is well worth it because there is no way to restore a barrel or chamber back to perfect condition, once it has started to rust. Therefore, the user should go through the extra effort if the ammunition is suspected to use corrosive primers.
As it turns out, the price of surplus ammunition using corrosive primers is often much lower than other types and it is also widely available in the market. So how does a user identify if the ammunition is corrosive or not? One way is by looking at the markings on the cartridges and the manufacturer of the ammunition. For instance, if it is surplus ammunition from certain countries such as the former Soviet Union, Yugoslavia or China, there is a good chance it uses corrosive primers, especially if it is manufactured before the 1980s. US-made ammunition has markings that can give a good clue as to whether the primers are corrosive or not. In case of doubt, there's an easy way to find this out. The user can take a cartridge, pull out the bullet from the front and empty out the powder from the cartridge and only leave the primer behind. Then. the user fires the empty cartridge onto a mild steel plate from a distance of about 1 inch from the muzzle, so that the primer chemicals are deposited on the plate. The user also fires another primer that is known to be non-corrosive onto another section of the steel plate. After firing the two cartridges, the user cleans the firearm as detailed in the procedure above, in case the suspect ammo is indeed corrosive. Then the user simply keeps the steel plate in a warm humid area for a few days. If the section where the suspect primer is fired over shows substantial rusting, then it uses corrosive chemicals.
Another way is to use bright common nails (which are nails made of mild steel with no coating) and pop a primers over one of these nails. If the nail rusts within a couple of days, then the primer is corrosive. The following video shows how this is done, using simple household tools:
Interestingly, the box of suspect ammunition actually says that it is non-corrosive on the box, but it turns out to be corrosive after all. Happy viewing!