Barrel Nitro Proofing - Quick Guide

What is Nitro Proofing? In The Shotgun Encyclopedia, John Taylor explains: "Before the invention of smokeless powder, guns fired black powder. Although black powder generates pressure that are quite low, the powder explodes rather than burns. Smokeless powder does not explode; it burns at a predictable rate, but reaches much higher pressure than black powder. The rate of burn is important in keeping loads within premissible pressure and manufactures formulate shotgun powders for specific applications." When nitro cellulose powder was introduced and even when it was widely accepted, Black Powder was still in use and firearms were still proofed for black powder. So a distinction had to be indicated. A firearm with Nitro Proof marks (the maker guarantees it's been nitro proofed) is a firearm that has been tested and fired with an "overcharge of nitro powder". Winchester used to do a 150% overcharge, while others would be considerably less. Who is CIP and what is their purpose? In a few words, C.I.P. is short from "Commission Internationale Permanente pour l'Épreuve des Armes à Feu Portatives" (The Permanent International Commission for the Proof of Small Arms) or C.I.P. is an International Standards Organisation which lays down Rules and Regulations for the proof of weapons and their ammunition. Most European countries that make guns, and Chile, are members of the CIP which lays down the standards for proof testing of guns and ammunition. USA, Japan, Turkey etc do not belong to the CIP. In this case you rely on the manufacturers' guarantee of pressure testing in factory. (No problems so far with USA and Japanese made guns. Time will tell about the others.)

In England, Europe and Chile, proof is the compulsory (regulated by the government) and statutory testing of every new small arm before sale to ensure, so far as it is practicable, its safety in the hands of the user.


Reproof is the similar testing of a previously proven small arm that has been rendered “out of proof” because one or more of the small arm pressure bearing component(s) have been materially weakened in substance or strength. There are defined internal bore size enlargement increments allowed for shotgun barrels, these can be found in the relevant Rules of Proof.


Both processes involve the strict gauging of key dimensions including the chamber, barrel and cartridge headspace dimensions. Each small arm is then visually inspected and only then, if suitable, will be tested with proof ammunition.


Proof ammunition, is designed to produce a minimum mean pressure uplift of between 25% and 30% above that of the maximum mean service pressure listed for the calibre, depending on the requirements. Such pressure should disclose weaknesses in the small arms: it is far better the weakness be found at a Proof House rather than during use, where serious personal injury may result. Barrel proofing in the US In the United States, there are no dedicated proofing houses and there are no legal reasons for them to exist. However, there are shops - like ours - that can test the barrels for those in need of this service.


The way barrel proofing is done in the US is similar to the way it is done outside the borders: by shooting through the firearm loads that are at 130% powder load, torching them off to make sure that an over-charge doesn't blow up the gun. There is a risk of blowing up the gun, of course, if the barrels cannot take the 130%. The point of this over-charge torching is so that if the firearm does show weakness and it cracks, the incident happens in a controlled environment before the owner can suffer any accidents. If the firearm cannot take the 130% loads, the firearm might be rendered irecuperabile. For firearms that are imported to the States from Europe, the barrel proofing is usually done in Europe prior to the import. Proof testing, actual proof testing pressure in England is 5.85 tons. European standard accepted in England is 5.72 tons Per/square inch. England/Europe use the long tonne which is 2240 lbs.

ENGLAND 13,104 PSI

EUROPE 12,812.8 PSI

What to do if you are not sure if your barrels have been proofed? If you are not sure if your barrels have been proofed, there are ways to find it out: the European barrel proofing houses are regulated by their local governments and they are required, once they have proofed the barrels, to mark them accordingly with a stamp. There are different proofing house stamps; the proofing stamp will tell which proofing house proofed the barrels, if they are indeed proofed, and also the time frame they have been proofed in, what loads they have been proofed for, etc. If there are any markings on the barrels, the proof marks can be found in an archive either online or by calling the proofing houses. The stamps would provide a very solid starting point. If there are no proofing marks, the barrels have not been proofed. If no stamps are present, take your firearm to a gunsmith (not an armorer!) and ask him to test the barrels for you, before shooting the firearm. Here is how to read the stamps on a barrel: A gun proof tested between 1896 and 1904 is examined below to illustrate the marks then used. It is a William Evans 28-bore with steel barrels, proofed in England.

This 28-bore is proofed under the Rules of Proof in force at the London Proof House from 1896 to 1904.

‘Smokeless’ powders were now widely used to the Britain. So, guns marked with these stamps show ‘NITRO PROOF’ unless they have been proof tested for black powder only.


1896-1904 Rules of Proof (London)


The stamps show: ‘V’ with crown over, which is the ‘View’ mark. ‘GP’ with crown over, which is the ‘Definitive Proof’ Mark ‘G’ in script with lion rampant over, which is the ‘Provisional Proof’ Mark ’28’ in a diamond indicates the chamber is 28-bore. ‘CHOKE’ indicates the muzzles of both barrels are choke bored. ‘3/4 oz MAXIMUM’ indicates the shot load. ’25’ indicates the actual bore size at nine inches from the breech.