Some manufacturers in the town of St. Etienne in France started conducting proof tests around the 15th century, when firearms manufacturing started in that area. However, France didn't enact a law to make the test mandatory until the 1900s and hence, it was left to each French manufacturer to decide what the test standards should be or even whether to perform a test or not. In other areas, such as London and Birmingham in England and Liege in Belgium, even before testing became compulsory, most manufacturers used to do private tests in their own factories or in a trade testing house.
Compulsory testing laws were passed in most countries mainly due to the backing of the gunmaker guilds of those countries. By enacting such laws, the guilds sought to prevent the manufacture of firearms and stifle competition from non-guild members, even though they claimed that such laws were designed to protect members of the public. The first such compulsory law was passed due to the lobbying efforts of the London Gunmakers Company (a guild composed of firearm manufacturers around London). The initial bylaws passed by the London Gunmakers Company charter of 1637 marked the first introduction of proof tests in England, but didn't specify an official standard of testing. By 1672 though, they managed to get an enhanced law passed, so that agents of the London Gunmakers Company could legally search premises and run proof tests on any firearms made in London or suburbs within 10 miles, or firearms made of imported foreign parts, or firearms brought into London for sale. In order to enforce a standard, the London Gunmakers Company established the London Proof House which was staffed by employees of the various manufacturers comprising the guild. It wasn't coincidence that most barrels failing the proof test requirements were those made by gunmakers who were not members of the London Gunmakers Company guild (to be fair, most of these were often of much lesser quality than London made guns). On May 10th 1672, Maximilian Henry of Bavaria passed a law that made proof testing compulsory in Belgium and established the Liege Proof House to enforce this law. In Birmingham, most reputable manufacturers had their own private proof facilities and many of them also made their facilities available for use by others, but since proof testing was not compulsory in Birmingham, cheaper manufacturers didn't bother to do this test. After a lot of lobbying by the reputable Birmingham manufacturers, the Birmingham Gunmakers Company was finally formed in 1813 by an act of Parliament and authorized to create its own Birmingham Proof House (which still exists to this day in the same historic building). The Birmingham Proof House conducted the exact same tests as the London Proof House, but had different proof marks to indicate that the tests were done in Birmingham rather than London. The firearms manufacturers in Birmingham benefited by this Act because it was more convenient and faster for them to send their firearms locally to be tested, instead of sending them all the way to London. However, the initial Act passed in 1813 proved insufficient, as many less-reputable manufacturers found legal loopholes to evade it, so another Act was passed in 1815 and in 1855, both of which were also ineffective. Finally in 1868, a suitable Bill was passed which remained in force for a while. This Bill was actually a private Act and not a Statute law and specified that the members of the Birmingham Company would elect their own guardians to enforce the proof test standards. It was also enacted that any person making or selling a barrel that was not proved in either the Birmingham Proof House or the London Proof House would be subject to a fine for each violation and any person forging the proof-marks would be subject to a fine as well and defaulting on payments would result in more severe punishments such as imprisonment, confiscation of assets etc. The English proof test procedures were amended in 1888, 1893, 1896, 1904, 1925, 1954, 1978, 1986, 1989 etc. to account for advances in firearms technology. Thus it became impossible to sell firearms in England that were not proved by either of these two proof houses. Oddly enough, there was no similar compulsory law enacted for any of the British Colonies around the world!
Even though Belgium had also made testing compulsory since 1672, the standards of the Belgian tests were far inferior to the tests conducted in England for a long time. In 1892, Germany also made proof testing mandatory and adopted a standard that was essentially the English standard with minor modifications. Since the German standard was so similar to the English standard, the German authorities automatically approved any firearms with English proof marks with no further testing, but rejected any firearms with Belgian proof marks until they were re-proven in Germany. Since Germany was a major export market for Belgian manufacturers, the Belgians promptly adopted the higher standards by 1893. During this period, there were proof houses in other European countries as well, such as St. Etienne in France, Wiepert and Ferlach in Austria, Budapest in Hungary etc., but as proof testing was not mandatory in these countries, marks from these proof houses were not as well regarded as those of Belgium, Germany or England.
The types of proof tests applied depended on the firearm being tested. Factors such as muzzle loading or breech loading, whether the barrel has rifling or not, bore of the barrel etc. all determine the type and number of proof tests to be run. In the case of multi-chamber firearms, such as revolvers, each chamber would have to be individually proof tested before the firearm was marked. Each proof house had its own marks based on the type of tests applied to the firearm. These proof marks would be stamped on to the barrel of the weapon, usually on the underside so that they would normally not be visible unless one disassembled the weapon for cleaning. The following picture shows some of the proof marks used by different countries:
Click on the image to enlarge.
In general, there are two types of proof test: The first is the provisional test which is done mainly on shotgun barrels and is done during the early stages of manufacturing, so that the gun-maker does not waste time continuing to build a gun if the barrel tube is defective. The second is the definitive test which is conducted on a firearm that is completely assembled (or close to completion). It tests the strength of the barrel and the firearm action together.
Depending on the types of tests, one could see multiple proof marks on a single weapon, such as the following:
From left to right, the different marks tell us several things about this firearm. First, the left most icon indicates that a Provisional proof test was run on it and this was administered by the Birmingham Proof House (because of the crown with stylized BP lettering under it). The next mark (12/1) tells us that the weapon is of 12 bore. The next two marks indicate that two additional definitive tests were run on it by the Birmingham Proof House, a Proof test (crown with BP) and a View test (crown with BV). The diamond shape with 12 C on it and the word "CHOKE" next to it indicate that this is a 12 bore weapon with choke-bored barrel.
The above image shows another weapon with proof marks left to right indicating: Provisional proof test run by London Gunmakers Proof House (Lion sitting on top of stylized GP), this is a 12 bore weapon (because of 12/1 mark), two more definitive proof tests were run by the London Proof House, a View test (Crown with V) and Proof test (Crown with GP). The diamond icon with 12 LC indicates that this is a 12 bore choke bored weapon and the "R. Choke" mark indicates that the barrel not only has choke-boring, but also has rifling in it.
The meaning of the proof marks for other countries were similarly organized, as the illustration below shows:
Indian Proof Marks
For example, the presence of a nitro-proof marking indicates that this firearm was also proved using newer smokeless nitro-powders.
These days, there are three main standards for proof testing: the C.I.P test and SAAMI test are for commercial firearms and the NATO EPVAT test for military firearms.
The CIP standard was established in 1914 in Liege, Belgium and ratified by law in 14 member countries (most of which are European) and it is illegal to sell civilian firearms in the member countries without having CIP proof marks from an accredited proof house. CIP also independently publishes data about ammunition dimensions, max. pressure generated by different ammunition types, max. pressure that can be tolerated by different firearms, testing procedures, compatibility between firearms and ammunition combinations etc. The current CIP member countries are Austria, Belgium, Chile, Czech Republic, Finland, France, Germany, Hungary, Italy, Russia, Slovakia, Spain, UAE and UK. In a standard CIP test, a firearm is fired twice with overloaded cartridges that produce 25% more pressure (30% more pressure for pistols, revolvers and weapons using rimfire cartridges) than the standard cartridge it would normally be fired with. After firing two overloaded cartridges, the firearm is disassembled and examined for magnetic flux leakage through fluoroscopic lamp in a dark room. If it passes, the CIP proof marks are stamped on to the metal, along with marks that indicate the date and the lab that performed the tests and the accompanying paperwork details are completed as well. Only then is the firearm sent back to the manufacturer or seller, who can now officially sell the firearm. Every civilian firearm that is for sale in a CIP member country is required by law to pass the tests, whether the manufacturer or seller is from a CIP member country or not. CIP also approves all ammunition that is sold by a manufacturer or importer in a CIP member country. The ammunition manufacturers are required to test each production lot in their factory against the CIP pressure specifications, document the test results and stamp each cartridge box with a CIP approved number that allows them to trace any quality-control problems back to a specific factory and lot number.
Similarly, in America, the SAAMI association (Sporting Arms and Ammunition Manufacturers' Institute) was founded by a group of American firearm and ammunition manufacturers in 1926 at the behest of the US Government and is an accredited standards developer for the American National Standards Institute (ANSI). SAAMI performs tests on weapons intended for the civilian market. SAAMI also publishes several technical documents, such as a list of Unsafe Arms and Ammunition Combinations which details cases where a smaller cartridge (e.g. a .44 magnum cartridge made by company X) could fit in a firearm designed to accommodate a larger cartridge (e.g. a .45 caliber pistol made by company Y), but would be unsafe to use because the cartridge produces higher gas pressures than what the firearm is rated for. Due to differences between SAAMI and CIP test procedures, there are some differences in pressure rating values for the same firearm and cartridge models and some combinations may be listed as unsafe in one standard and safe in the other. One more difference is that while the SAAMI association requires its member companies to follow the guidelines and product standards that it sets, it is not a compulsory standard enforced by any branch of the US government (unlike the CIP tests, which are required by law to sell a firearm or ammunition in a country that is a CIP member). All major American ammunition manufacturers are members of SAAMI and most smaller companies also follow its guidelines, except for a few smaller manufacturers. SAAMI also has other committees, such as one that works with CIP to develop common international standards, wildlife conservation, standards for transport and regulation of firearms etc.
The NATO EPVAT test standard is for military grade firearms. This is a much more comprehensive set of procedures than CIP or SAAMI and uses much more sophisticated test instruments.
Proof tests were instrumental in protecting the end-user from weapon failures and compulsory enforcement of these laws helped reduce firearm-related accidents around the world. This is why they are still mandatory in many countries.