Just for everyone's interest, and before everything disappears from PictureTrail
there are rust inclusions
within the barrel wall (not upon the surface
as these appear to be) called "greys"
Austin Hogan's Parker 0 Grade Lifter with tiny rust inclusions in the Plain Twist barrels
Peter Hawker, Peter William Lanoe Hawker, "Instructions to Young Sportsmen in All that Relates to Guns and Shooting", 1859 discusses the dangers of that new-fangled (probably Bessemer) steel stuff and mentions "greys"
http://books.google.com/books?id=yXUCAAAAYAAJ Barrels made entirely of steel are now much in fashion with some of the gunmakers, who are glad to catch at any new “dodge” that will add grist to their mill. The material for this new manufacture is, for the most part, collected from old coach- springs, which are cut in slices like a cucumber, by a 45-horse power engine that works the colossal shears, to which little boys apply the metal with a rapidity like magic. The steel, after being smelted to a bloom in the air furnace, and beat by a 3Ł tons' steam-hammer, works much cleaner than stubs or any other iron, and is therefore more profitable, because so little of it is cut to waste, when boring to get rid of holes or flaws that are called “
grays.” In short, it is a metal better for trade, and therefore all parties have an interest in recommending it, and tell you that it kills the birds much better than the good old horse-nail-stubs, that are now so scarce as to be almost obsolete.
This assertion, I admit, may have some truth in it, because we know, the harder a barrel is, the stronger it will shoot.
But then we must remember another thing —
the more likely it is to burst! I have no objection to keep pace with the times so far as what the coalheavers call “half-and-half” goes — half stubs and half steel; but, knowing that, if steel should give way, it may blow your head off, instead of merely bulging the barrel, I decline what is vulgarly called “going the whole hog” for fashion's sake; and, therefore, I desired Clive, the best barrel-maker in Europe, to forge a large barrel with stubs and steel...
James Dalziel Dougall, "Shooting Simplified", 1865
There are various kinds of flaws in barrels, such as 'cracks', 'sand holes', and what are technically termed '
grays'. They are those little specks in the iron from rust showing more upon them than upon the smoother surface.
W.W. Greener "The Gun" 1907 edition
One of the greatest difficulties with which a gun-maker has to contend is the "
grey" in gun barrels. The "grey" is a defect of small actual importance, but decidedly a blemish on a fine weapon and an eyesore in every description of gun barrel. The numerous twistings and weldings of gun-iron rods and ribands are fully detailed in the description of the barrel-welding processes, and it must have occurred to the reader that the Damascus barrel is one mass of welds from breech to muzzle. This is so. Unfortunately a certain amount of burnt metal, or scale, is imbedded within some of these welds, and in the finished barrel this fragment of scale forms a "grey," or small speck of useless material, which will not colour in harmony with the other part of the barrel, but is made more apparent by the finishing processes of polishing and browning. These “greys” may appear some time after the gun has been in use, the hard metal composing the barrel being eaten into by rust, or the thin coating over the “grey” being worn away. They are developed in the inside by the chemical action of the powder gases, and are practically ineradicable.
Sportsmen must
not imagine that “greys” weaken a barrel to any appreciable extent, and their development in a gun, after some months' or some years' wear, in no way reflects upon the reputation of the gun-maker. A barrel eaten right through with rust, at or near the muzzle, may be fired with perfect safety; consequently a "grey" is not to be regarded as an element of danger; and
barrels after thirty years' wear, or after firing upwards of 100,000 shots, are safe to use, providing they are free from dents, bruises, and rust inside.