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Proof test

A proof test is a test wherein a deliberately overpressured round is fired from a firearm in order to verify that the firearm is not defective and will not explode on firing. The firearm is inspected after the test, and if it is found to be in sound condition, then it is marked with a Proof Mark to indicate that is has been proofed (not proven). In many jurisdictions a proof test and valid proof mark are required for the sale of firearms.

A proof round is an ammunition assembly designed to be used in proof testing; this can use a fixed cartridge, a semi-fixed cartridge, or separately loaded projectile, charge and primer. A proof shot is a special projectile used in a proof round or other projectile weapons, electromagnetic guns for example. Small arms proof rounds resemble normal cartridges, though they will typically bear special markings to prevent them from being confused for standard cartridges. Large calibre arms, such as artillery, will in general use an inert solid projectile (the proof shot); although water, sand or iron (powder) filled versions can be found for testing recoil systems.

Testing methodology

For both small arms and heavy weapons, the gun is fired remotely and then examined; if undamaged, it is assumed to be safe for normal use and a proof mark is added to the barrel. In the case of revolvers or other multi-chamber firearms, each chamber must be proof tested before the firearm may be marked. Examination of the firearm may be as simple as visually inspecting it (defective components may fail in a spectacular manner, resulting in an explosion of the firearm) or may involve more in-depth examination, at the option of the tester.

Because proof testing may or may not result in the destruction of the test specimen, it falls on the border between destructive testing and nondestructive testing.

Proof marks

A proof mark is a mark specific to the manufacturer, importer, or testing facility that is performing the test. It generally takes the form of a stamp that makes an impression in the metal. Since proof marks are unique and nearly universal, they are often used to identify the origins of firearms that lack normal manufacturer's markings, such as military weapons, which are often produced by large numbers of different manufacturers.

Small arms proof testing

A small arms proof round is loaded to a higher than normal service pressure, stressing both the gun barrel and breech during firing. This can be due to a heavy projectile fired using the standard propelling charge, the standard projectile fired with a different propellant type or weight, or combinations of charge and bullet weight to give the required proofing pressure. Minimum proof testing pressures are specified by the owner of the cartridge specification, such as C.I.P. or SAAMI for most commercial cartridges or NATO EPVAT testing for appropriate military cartridges.

An example proofing round for the 0.5" Browning HMG (12.7 x 99 mm) is the 'Cartridge, Calibre, .50, Test, High Pressure, M1'. This uses the standard weight 0.5" M1 round propellant (240 grs of WC860), but a bullet weighing 999 grs (+/- 11 grs). The M1 proof round gives a proofing pressure of ~65,000 psi, 11,000 psi (~17%) above the standard service pressure.

Proof testing in C.I.P. regulated countries

In C.I.P. member states every civil firearm has to be professionally proofed in accredited Proof Houses before it can be sold to consumers.

The standard proof test consist of firing two overloaded cartridges that produce 25% more chamber pressure then the C.I.P. specified maximum pressure limit for the same cartridge in its commercial version. The standard proof of pistol, revolver and rimfire cartridges is performed with overloaded cartridges that produce 30% more chamber pressure than the C.I.P maximum pressure limit for the same cartridge in its commercial version. There are only two overloaded firings to avoid excessive stress to the arm, especially the barrel which is the main part suffering this overload beside the chamber (when not part of the barrel) and the locking mechanism. After the test, the arm is disassembled by the proof house technicians for nondestructive testing looking for Magnetic flux leakage through fluoroscopic lamp in a dark room.

Before the year 2006 the standard test consisted of firing two overloaded cartridges producing 30% more chamber pressure then the C.I.P. specified maximum chamber gas pressure limit for the same cartridge in its commercial version.

Voluntarily testing beyond the current legally required standard test benchmark is often also possible for consumers who intend the use their firearms under extreme conditions (hot climates, long strings of shots, etc.). In case a firearm passes sutch a proof-test a pass mark termed superior proof mark is stamped in every successfully tested firearm.

Proof test differences

Under SAAMI proof test procedures, for bottlenecked cases the centre of the transducer is located .175" behind the shoulder of the case for large diameter (.250") transducers and .150" for small diameter (.194") transducers. For straight cases the centre of the transducer is located one-half of the transducer diameter plus .005" behind the base of the seated bullet. Small transducers are used when the case diameter at the point of measurement is less than .35".

Under C.I.P. proof test standards a drilled case is used and the piezo measuring device (transducer) will be positioned at a distance of 25 mm from the breech face when the length of the cartridge case permits that, including limits. When the length of the cartridge case is too short, pressure measurement will take place at a cartridge specific defined shorter distance from the breech face depending on the dimensions of the case.

The difference in the location of the pressure measurement gives different results than the CIP standard.

Large caliber proof testing

The test of a large-caliber weapon system primarily covers the strength of the barrel, breech and recoil systems. The proof shot has to resemble the resistance to motion (bore/rifling friction, shot start pressures, etc) and profile to the propellant gases that the actual service projectile will give. For this reason, proof shots for APFSDS rounds have fin units and all must have profiles behind the obturator the same as the shot they are emulating. Crack analysis is often undertaken on large-caliber weapons after testing has been carried out.

The proof shot is normally a high-drag projectile since its job is over once it has left the muzzle of the gun. A high-drag projectile is advantageous for two reasons; first, it reduces the impact velocity when fired against an earth or sand backstop, and second, it reduces the range if no backstop is used. Excessive range can be a very problematic when firing any large-caliber round; safety traces can often exceed the bylaw areas of the firing range, so range reduction is imperative. This is even more of a problem when high velocity, low drag rounds such as APDS or APFSDS are used.

It should be noted that although proof shots are used in the service proof of barrels before use, the vast majority are used in propellant charge design. Proof shot emulating APFSDS rounds are also used in stabilizing-fin SoD trials, which is often combined with a charge design trial.

Instrumentation used in Proofing

When running a charge development, or Strength of Design (SoD) trial, the charge mass and service pressure will gradually be worked up to the required proofing pressure of the weapon system. Readings will be taken of chamber pressure by copper crusher, or piezo electric gauges and velocity by Doppler radar (in-bore or aeroballistic), or photocell counter chronographs. In addition strain and temperature readings may also be recorded. If required, high speed photography (synchroballistic photography, high speed digital stills, head on cine, or flight follower) may also be used.