Bouching the guns: Preventative against vent erosion

The term “bouching” comes up from time to time in reference to Civil War artillery.  The practice dated back practically to the invention of the cannon, or at least in written records to the 18th century.  Since the metal around the gun’s vent tended to erode faster than other surfaces, bouching extended the service life of the cannon. In the Artillerist’s Manual, Major F.A. Griffiths, Royal Artillery, defined the term:

Bouching a gun is fixing a pure copper vent into it; which is done by drilling a hole in the piece, where the vent is usually placed, about one inch in diameter, and screwing therein a piece of wrought copper with a vent of two-ninths of an inch through the center of it.

A 12-pdr Napoleon, registry number 90 from Henry N. Hooper & Company, representing the 9th Massachusetts Artillery at Gettysburg’s Trostle Farm, provides a good example of such a bouch.

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Vent of 12-pdr Napoleon, Hooper #90

On this side of the Atlantic, John Gibbon’s Artillerist’s Manual provided a justification of the practice, but introduced an alternate terminology:

In bronze pieces, the heat and current of gas through the vent melts out and carries away the tin, rapidly widening the aperture.  For this reason the vent is made in a vent-piece of pure copper screwed into the gun, which enables us, when the vent becomes too much enlarged, to replace it by another.  When the charges used are very small, and the projectile not very heavy, no vent-piece is used.  In iron pieces the vent is made in the metal itself.

However with the introduction of rifles, even the iron guns suffered from rapid vent erosion.  I’ve mentioned in the past that 4.5-inch Ordnance (Siege) Rifles suffered due to the lack of bouching.  The Army specified that Phoenix Iron Company bouch the smaller 3-inch Ordnance Rifles before delivery.  A common sight on those guns is a slightly off color circle of metal around the vent. And often enough, the vent-piece shows sighs of erosion.

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Vent of 3-inch Ordnance Rifle #670

Less common, but encountered enough for notice, some guns are missing the vent-piece today.

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Vent without Vent-piece on 3-inch Ordnance Rifle #737

In the case of number 737, there’s a squared off section, possible due to erosion, that narrows down to the chamber.

This may seem like a trivial detail.  But the vent was arguably the weakest point of the gun.  If the vent enlarged, the primer would not seat properly.   An enlarged vent also allowed excess gas to escape, reducing the gun’s performance.  The Army’s Ordnance Manual called for vent repairs when the diameter exceeded 0.3-inch.  For guns with a vent-piece, the repairs involved unscrewing the eroded vent-piece and replacing it.  For older guns without the vent-piece, the manual required the old vent filled with zinc.  A new vent was then drilled a few inches away from the original.

But thinking back again to the guns on the battlefields today, many of them have vents within the 0.3-inch tolerance.  I don’t think we can directly attribute that to the strength of the metal.  Perhaps more so to the thrift of a post-war Army.