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Thin Barrels

Thin Barrels.

‘Barrels below recommended minimum’ is a line often seen in an auction catalogue description of guns for sale. What is that minimum and what does it mean to a buyer?


Proof marks will help you determine the original bore size. In this case ’12’, which indicates .729″ at 9″ from the breech.

The proof laws do not state a minimum barrel wall thickness to denote the legal status of a gun. They are interested primarily in the bore diameter to determine proof status. The wall thickness measurement is one the trade keeps a keen eye on, in order to calculate the likely strength and ‘life’ in the barrels.

The approach may seem odd but it makes sense. It allows for technology to produce better steel with greater strength at reduced thickness. If the barrel passes the stress test, it is deemed fit for purpose. If it fails, it is not. Thickness becomes a redundant matter.

In second-hand guns, particularly those over a hundred years old, wall thickness is one of the most crucial means of determining value. Many will have started life with damascus barrels of significant thickness. A minimum of 50 thou’ is not unusual. When assessing a gun of this type the original bore dimension is a matter of importance and being able to read the codes correctly is useful.

For example, a 12-bore barrel stamped ’13’ is a nominal 12-bore but was made with a bore diameter of .710” rather than the actual 12-bore measurement of .729”. Subsequent re-proof may tell you that the bores were proof tested at .729” in 1935 and again at .740” in 1970. This indicates the bores becoming enlarged due to internal lapping to remove rust and pitting. If your minimum wall thickness is now 20 thou’ on the gun in question, we can determine the original thickness from the proof marks. In this case, it would have been 50 thou’.

The ‘recommended minimum’, to which the catalogues refer, is 20 thou’. Any less and many gun shops will not sell the gun to the public. This is why many such guns appear in auctions.

Re-browned barrels on 1874 Purdey.

Externally, barrels may look very good, but measuring the wall thickness is essential.

To complicate matters, some guns were made with barrels originally thinner than the current ‘recommended minimum’. This was necessary to achieve the weight and balance specified when meeting the order. For example, if a 12-bore is to be made to a weight of under 6lbs with 28” barrels and a 14 3/4” stock, the barrels have to be struck thinner than if the specified weight is a more normal 6lbs 12oz. A Purdey sold at Holt’s in June illustrated this well. It was built to the normal Purdey game gun weight of 6lbs 11oz but was ordered with 32” steel barrels. The bores still measure the dimensions with they originally left the factory and the walls are 18 thou’ and 19 thou’.

Being able to determine whether a gun barrel had thick walls originally and now has much thinner walls due to repairs, or whether it was made with thin walls and retains its original integrity, is one of the finer points of appraising a gun. Thin walls are usually an indication of wear and repair but not always. The devil, as they say, is in the detail and the bare facts do not tell the whole story.

Buying a gun with thin walls (perhaps 18 or 19 thou’) can be a way of beating the market, as the price will be significantly lower than the same gun with better wall measurements. It can be a means by which you can shoot a best quality sidelock for the price of a decent boxlock. If cared for, the gun will be safe and last your lifetime. However, be prepared for the on-going re-sale value being relatively low and the propensity to dent being higher: so keep the gun forever and be careful when transporting and storing it.


Thickness at the muzzle is not much use, as the thinnest section will be several inches back.

Once walls get down to 17 thou’ especially if not made this way and even more so if the thin spots are localised and sudden changes, rather than smooth transitions to a uniform thinning around the whole tube, be very careful, as the barrels are really past their useful life and will need to be sleeved.

One can see a lot of beautiful guns in auctions, and when tempted by one with thin barrels, take stock and do your calculations. However pretty the gun is externally, however mechanically sound and nicely engraved, sleeving will cost at least £1,500 and on a best gun will require £3,000 to do it justice. The value of sleeved guns is limited and always lower than those with original barrels in good shape, so the investment may be hard to re-coup.

Where it can work in your favour is when looking for a gun as a project to sleeve to a barrel length you especially want, with chambers and chokes also customised. In this respect, the vintage gun with thin barrels may be just what you need.