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Barrels, In proof or out of proof?

A lot of confusion surrounds what makes a gun barrel either in proof or out of proof. Even if its out of proof, it may not be the end of that barrels usable life.

When a gun barrel is initially proofed, the barrel will have a given bore diameter. This measurement will be marked on the barrels when the gun barrel goes through proof, the measurement is taken at 9 inches from the breach (shorter in some proof houses, like Liege who take the measurement at 15cm (just under 6 inches)).

For example a true 12 gauge is 0.729 inches or 18.4mm, however many barrels are initially proofed both smaller and larger than this measurement. The guns proofed smaller than this are often older guns or continental. Newer sporting/clay guns are often larger than the true gauge to reduce recoil, 0.732 or 18.6mm, basically the cartridge has more space so pressure is reduced vs a tighter bore.

A gun barrel becomes ‘out of proof’ when the bore diameter is 10 thou greater than what it was initially proofed at. So a gun proofed at 0.729 inch is out of proof when it gets to 0.740 inch, the bore has worn by more than 10 thousands of an inch during its life. This sounds like the end of the world if it is your gun and unfortunately it can be, however, if the wall thickness in the barrel is still sufficient at 6 or 9 inches (minimum 21 thou, but ideally 30 thou or thicker), you can still re-proof your gun and have it re-marked with the diameters at that point of proof. If the bore diameter goes above 0.744 inch in a 12 gauge, it will be marked as ‘overbore’. Going through a re-proof means your gun barrel has passed the safety test and it is legal to sell on if you so wish.


The table below is the minimum/maximum bore diameters as per the CIP guidelines. Any barrel below the minimum may be able to receive local proof marks but will not be stamped CIP as it will not meet CIP guidelines.


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