Brybenn, 9.3 Mauser is on the right track. British guns all underwent proofing before initial sale and even used guns must be within the specification of the latest stamped proof. To sell an out of proof gun in England or Europe is still illegal. In Canada and USA where we never required government controlled proofing each manufacturer is responsible for their own proofing on new firearms, but to no recognized standard. Go figure. Your beloved vintage Parker or Fox or Purdey or Greener could have been bored out in this country to remove pitting to the point that the barrels are paper thin and most prospective buyers either don't know or don't care.
British proof is a large and complex subject and the requirements changed quite a few times as new advances such as choke boring, smokeless nitro powders and different case lengths came along. The markings were changed each time the parameters were changed, sometimes very subtly and at least once these changes came three times within ten years. Proof is a very complex and misunderstood subject, here are a few basics regarding British proof, European proofs use different markings but similar standards.
British proofing has been in operation for over 250 years. The original purpose was to weed out substandard and dangerous barrels.
The system is based on black powder standards even though the standard proofing today is a nitro proof for modern ammo. Our ammo still follows this system, eg 2 2/4 or 3 dram target loads, this is the 2 3/4 or 3 dram black powder equivalent.
The British system presumes a standard black powder equivalent for all loads up until recently when they changed over to the CIP European standard.
The pre 1875 proofs gave the bore size but no indication of appropriate ammo
By 1875 choke boring was becoming common so choke could be indicated as 12 B, 14 M meaning a 12 bore with a 14 bore muzzle.
In 1887 the weight of the proof load of shot was included, such as 1 1/8 , 1 1/4 or 1 3/8 etc. Using a table this referred to 2 1/2", 2 3/4" or 3" ammo. All of these were at the same black powder equivalent and velocity, an increased powder charge driving a heavier shot load but at the standard velocity and pressure.
In 1896 Nitro Proof first shows up because the new smokeless powders had different burning characteristics and were damaging some otherwise in proof guns. From this point until now guns marked Nitro Proof, NP, BNP were designed and each one proof tested with a special proof overload to be safe with modern ammo. All guns were now getting provisional black powder proofing and most were also nitro proofed However not all guns received Nitro proof, it cost a little extra and was sometimes omitted to save a little money, especially for guns intended for export to North America where nobody knew or cared what proof was. British Nitro proof did not become compulsory until the 1920's, there are some guns around that appear modern but were never nitro proofed.
Eventually in the early 1900's the actual case length markings were added, first in inches, later in millimeters. Also actual bore size was added.
So there are many (most) vintage British shotguns with no markings of gauge or suitable ammo unless you can decipher the proof hieroglyphics. Ditto markings regarding the intended ammunition. The British ammo was all produced to the same pressures and velocity, regardless of case length. They had a 12 gauge 3" and a 10 gauge 3 1/2 " in the 1800's but they were not at higher magnum velocities or higher pressures like North American ammo, they just delivered a heavier payload. Modern North American ammo can be higher pressure than these guns were designed and proofed for and it is very hard to get the manufacturer's actual pressures for any given load. Never assume anything, some heavily used target loads, while of modest ballistics actually produce maximum North American pressures. Personally I use Gamebore or RST with published modest pressures in my vintage guns and of course my reloads where I can control the intended pressures.
Also of note, because an out of proof gun can't be sold in Britain or Europe they are sometimes exported to countries like Canada or USA where there are no proof laws, few people can properly measure barrels, nobody knows, nobody cares. EXCEPT on valuable guns, even here knowledgeable buyers insist on in proof guns when the price goes up. Nobody is buying out of proof Purdeys or Holland and Hollands.
