Understanding UK Proof Marks and What They Mean for Vintage Guns

Pick up a British side by side, turn it over, and on the flats you will see a tiny archive of its life. Crowns, letters, numbers, sometimes a fleur de lys. Those stamps are more than decoration. They certify a shotgun’s fitness, show where it was tested, reveal how it should be fed, and help you date and value it. If you shoot or collect vintage British guns, being able to read proof marks is part safety, part law, and part detective work.

At Game & Gun we spend a great deal of time measuring bores, checking wall thickness and interpreting marks on London and Birmingham guns, along with work on Continental pieces that later entered British trade. This guide distils what matters most, so you can make good decisions with confidence.

What proof really means

British proof is a legal test and a permanent record. A shotgun is subjected to an over-pressure charge at an authorised proof house, inspected, then stamped. Since the Gun Barrel Proof Acts of the 19th century it has been illegal to sell a gun in the UK without valid proof. That remains true today.

Two institutions carry out British proofing:

• London Proof House, operating since the seventeenth century
• Birmingham Proof House, created in the early nineteenth century

Both apply marks that indicate the type of test and the standard achieved. Since the UK adheres to CIP rules, modern proofs follow European pressure standards and you will often find markings that reflect this shared system.

A key point for owners is that proof is not a one time abstraction. If you later alter barrels, lengthen chambers, add choke, or make other material changes, the gun must be reproofed before it can lawfully be sold. The stamps you see are therefore both a certification and an instruction sheet for future custodians.

Where to find the stamps and how to read the layout

On most British shotguns, the relevant marks sit on the barrel flats, the action flats, and sometimes under the top rib. You may also see view marks on the action face. Typical elements include:

• A crown above letters that denote the house and test type
• Nitro or black powder designation
• Chamber length in inches or millimetres
• Proof pressure or load information for a given era
• Date codes or inspector symbols
• Gauge or bore data relevant to the period rules

Reading order matters less than understanding the eras and the language used at the time your gun was tested. British proof changed with propellants, gauges in common use, and international standards. Learn the broad eras first, then the fine detail gets easier.

A quick timeline that helps everything click

• Before the 1890s: Black powder proof. Early marks and view stamps, often without modern chamber or pressure notation.
• From the late 1890s: Nitro proof introduced for smokeless powders, alongside marks that begin to specify shot charge and chamber data.
• 1925 to mid century: Chamber length appears as a matter of routine, usually in inches on British pieces, sometimes metric on imports.
• 1954 onwards: You will often see “tons per square inch” on British nitro proofs, for example 3 tons or 3¼ tons per square inch on 12 bore guns.
• Late 1980s onwards: Shift to CIP bar markings. You may see 850 bar for standard nitro proof on many shotguns, and other values for magnum or superior tests.
• High performance steel shot proof: Look for the fleur de lys symbol alongside pressure designations that show the gun can handle HP steel cartridges. Without the fleur de lys you do not have HP steel proof.

Common British marks you will meet

The following table is not exhaustive, but it covers most of what you will see on London and Birmingham proofed shotguns from the black powder era to present.

Mark seen on the flats or action	Era or house	What it tells you
Crown over GP	London	Gunmakers’ Company proof mark, seen with London view marks. Confirms London proof.
Crown over NP	London	London nitro proof, smokeless powder standard for the period of test.
Crown over BNP	Birmingham	Birmingham nitro proof, smokeless powder standard for the period of test.
V under a crown	London	View mark after inspection, applied alongside other London stamps.
Crossed swords with letters	Birmingham	Inspector’s date code device used by Birmingham. Letters indicate year code.
BP or crown over P	Various black powder eras	Black powder proof. Treat as black powder only unless later reproofed for nitro.
CIP and “N” or bar number	Modern	CIP nitro proof under European pressure rules, usually paired with a bar figure.
12, 16, 20 etc	All eras	Gauge. Context matters, older guns may use different styles to show bore.
2½, 2¾ or 65, 70, 76	From 1925 onwards	Chamber length in inches or millimetres. 65 mm is 2½ inches, 70 mm is 2¾ inches, 76 mm is 3 inches.
TONS per square inch	c. 1954 to late 1980s	British pressure indication prior to wholesale shift to bar. Often 3 or 3¼ tons for 12 bore.
850 bar, 1200 bar, 1370 bar	Late 1980s onwards	CIP service pressure for the class of proof. 850 bar is common for standard shotgun nitro proof.
Fleur de lys	Modern	High performance steel shot proof. Required for HP steel loads. Not present means no HP steel proof.

Context matters. A crown over BNP with 70 mm and 3¼ tons per square inch is a different story to a crown over NP with 65 mm and no tonnage, even though both are nitro proof. Look at the whole set, not a single stamp.

How proof affects legality, use and value

• Sale: In the UK, a gun without valid proof cannot be sold by a dealer or privately transferred. If it has been altered since proof, proof is considered void and reproof is required before sale.
• Use: The law focuses on sale and transfer. Using an out of proof gun is unwise and may void insurance. Proof exists for safety, so treat out of proof status as a stop sign.
• Reproof: If you lengthen chambers, thin barrels through heavy honing, remove and re-lay ribs, or make any material change, plan for reproof. Budget both the work and the proof fee.
• Value: A clear, original and coherent set of British proof marks boosts confidence. Gaps, abnormalities or erased areas reduce value and increase risk. Reproof can maintain value when done transparently and sympathetically.

Proof marks also shape a gun’s audience. A 12 bore with 65 mm chambers and light nitro proof will appeal to the game shooter who enjoys traditional loads. A 12 bore with 76 mm chambers and fleur de lys proof reaches a different buyer who wants HP steel loads. Both are fine, they simply belong in different roles.

Steel shot, vintage barrels and what the marks actually allow

This is a topic loaded with half-remembered advice. Accurate reading of proof marks and a proper measurement session resolve most of the noise.

• High performance steel: Only for barrels marked with the fleur de lys and the appropriate modern pressure marks that confirm HP steel proof. Without the fleur de lys, do not assume HP steel is acceptable.
• Standard steel: Many nitro proofed guns with 70 mm chambers and sensible choke can use standard pressure steel loads, but only after confirmation of choke, bore size and wall thickness by a competent gunsmith. Tight full choke, very thin muzzles or very early steels can be a poor mix.
• Black powder or early nitro: Black powder proof means black powder only unless later reproofed. Early nitro proof with short chambers and thin walls is best kept on traditional lead game loads at pressures within the era’s envelope.

Always treat proof marks and real measurements as a pair. Proof shows what was passed on the day. Measurement tells you what exists today.

Misreadings that catch people out

• 12 65 does not equal 12 70: A 12 bore marked 65 mm chamber is not a 2¾ inch gun. Do not use 70 mm cartridges in it unless it has been lengthened and reproofed, or the maker originally cut it for longer shells and the marks reflect another system. Assume the mark is correct until proven otherwise.
• BNP is not a steel stamp: Birmingham Nitro Proof is smokeless proof. It is not a steel shot approval in itself.
• Tons versus bar: 3 tons per square inch belongs to a mid century British scheme. Do not try to compare it directly to modern bar numbers without a proper table. Treat the gun by era and service it was built for.
• Foreign marks in the mix: Many British trade guns used Continental tubes or were partially finished abroad. An ELG in an oval shows Liège proof. If you then see later British proof marks, interpret them together and use the newest relevant nitro proof for your decision making.
• Reproof wiped by refinishing: Heavy polishing and reblacking can erode shallow marks. If stamps look washed out or incomplete, be cautious and document what remains.

A practical checklist before you buy or shoot a vintage British shotgun

Step	What to Check	Details/Notes
Confirm house and type	Proof house (London/Birmingham), proof type (nitro/black powder), foreign proof marks	Identify all proof marks and their origin
Read chamber length	Chamber length in inches or millimetres	2½” = 65mm, 2¾” = 70mm, 3″ = 76mm
Note pressure system	Pressure rating units	Tons per square inch or bar; record exactly as stamped
Look for steel proof symbols	Presence of fleur de lys or other steel proof symbols	Note any additional wording regarding steel
Inspect for reproof	Multiple sets of proof marks	Latest valid proof governs use
Measure the barrels	Bore diameter (9″ from breech), choke (each muzzle), minimum wall thickness	Use proper gauges for accurate measurement
Check for material changes	Chamber lengthening, choke alteration, rib relaid, sleeving	Look for evidence of modifications
Decide the role and feed	Cartridge and quarry suitability based on proof status and measurements	If uncertain, use conservative loads or consult a specialist

Two worked examples

Example 1, a 1936 Birmingham boxlock:

• Marks: Crown over BNP, 12, 2¾, “3¼ TONS PER SQUARE INCH,” inspector device with crossed swords and date code.
• Read: Birmingham nitro proof for 12 bore, chamber length 2¾ inch, mid century tonnage system. Set up for standard game loads of the era and many modern lead cartridges of moderate pressure. Not steel HP proof, so avoid HP steel loads. Standard steel may be acceptable after measurement if chokes are not overly tight and walls are sound.
• Next steps: Measure bores and walls, pattern with well chosen lead cartridges. Maintain with light honing if needed, avoid unnecessary metal removal.

Example 2, a late Victorian London sidelock later reproofed:

• Marks: Early London view and proof marks, 12, 2½, crown over NP added in the early 1900s, later bar marking 850 with a fleur de lys in the modern set.
• Read: Originally black powder, then early nitro, later reproofed under CIP with HP steel proof. The latest marks, including the fleur de lys, govern. If barrels measure well, the gun is cleared for HP steel under modern rules.
• Next steps: Verify measurement, pay attention to choke profile. Enjoy the flexibility from the modern proof while respecting the mechanics of a Victorian action.

These examples show why it pays to read the whole story, not just a single stamp in isolation.

When proof is missing, muddled or out of date

No marks or marks that have been obviously removed are red flags. Guns outside British proof should be treated as unproofed in the UK market, which restricts sale and usually depresses value. Sometimes a gun has full Continental proof but never received British proof after import, which may also block sale until a British house tests it.

Reproof offers a route forward. Done properly, it restores legal status and reassures the next owner. The craft side matters here. Barrels must be mechanically right before they will pass. Ribs must be secure, dents raised properly, bores trued without weakening the tube. A clean bill of health and a fresh set of stamps transform a gun from a nervous buy into a trustworthy field companion.

How we approach this work in the workshop

Game & Gun is set up to handle vintage British shotguns to an exacting standard.

Measurement and diagnosis

• Bores, chokes and minimum wall thickness measured on calibrated gauges. Written records provided.

Mechanical rectification

• Raise dents, strike off judiciously, re-lay ribs and solder work, tighten actions, time ejectors, correct striker protrusion and spring strength.

Sympathetic refinishing

• Hot or cold bluing as appropriate, careful polishing that preserves edges and preserves remaining marks, wood repairs that keep original lines.

Parts made in house

• Springs, pins, sears, extractors and levers fabricated to pattern, with jigs to replicate geometry on sidelocks and fine London actions.

Preparation for reproof

• We prepare barrels to pass, test fit with correct bite and bite depth, and advise on sensible load choices after proof.

This careful approach is not about shortcuts. It is about getting it right the first time, so the next inspection stamp is a formality rather than a gamble.

Subtleties that interest collectors

• Transitional marks tell stories. A London gun with Belgian tube marks and later Birmingham reproof can reflect the realities of the trade and big London names using Continental barrel makers.
• Rare inspector devices and date codes can increase appeal. Always authenticate with reliable charts and remember that refinishing can blur fine detail.
• Proof against nominated powders appears on some interwar guns. Treat these as snapshots of the test protocol of the day, not a fixed ammunition requirement now.
• Sleeving and rebarrelling are not sins, they are chapters. If done by a respected maker and correctly reproofed, value and use can both remain strong.

Quick reference tips you can keep in your pocket

• Crown over BNP, 70 mm, 3¼ tons per square inch: mid century Birmingham nitro proof for 2¾ inch cartridges.
• Crown over NP, 65 mm, no tons figure visible: London nitro proof for 2½ inch cartridges, likely earlier nitro period, keep loads light and traditional unless later reproof shows otherwise.
• 850 bar with no fleur de lys: standard modern nitro proof, not HP steel proof.
• 12 over 70 in a rectangle, CIP mark nearby: modern metric chamber marking for 12 bore with 70 mm chamber under CIP rules.
• Fleur de lys present: HP steel proof, but still check choke and cartridge specifications as marked by the manufacturer.

When to ask for help

If a mark is faint, a date code unfamiliar, or the measurements are borderline, ask a specialist. The cost of a proper inspection is small compared with the cost of a mistake. We routinely assist clients with:

• Pre purchase inspections for British boxlocks and sidelocks
• Verification that a favourite family gun is suitable for the cartridges they want to use
• Preparation of barrels so that a planned reproof becomes a predictable process
• Sourcing or making parts for London pattern locks and complex ejector systems

For those who like their kit to match their field days, we also stock footwear and apparel that stand up to bad weather and long drives, along with mounts and accessories that keep modern optics aligned with classic guns.

A final word of caution and confidence combined. Proof marks are the gun’s own paperwork, pressed into steel. Read them, measure well, and work with people who respect what they mean. That way your vintage gun will carry on doing what it was built to do, safely and with grace.