TOPIC:

12-pounders of the RHA

Source: www.angloboerwar.com/forum/19-ephemera/3...n-jack?start=0#90162

Naval field batteries

Source: www.angloboerwar.com/forum/19-ephemera/3...n-jack?start=0#90162

Gun detachment of the Cape Mounted Rifles
The CMR are gunners as well as cavalry and infantry

Source: www.angloboerwar.com/forum/19-ephemera/3...under-the-union-jack

Preparing for shore service
Transferring a ship's light gun to lan mountings

Source: www.angloboerwar.com/forum/19-ephemera/3...n-jack?start=0#90162

For some time now I have been trying to get my head around the unusual narrow wedge-shaped rifling ridges found on some driving-band fragments brought home as souvenirs from South Africa.



Close-up of the groove pattern on one of a pair of napkin rings. Each has an applied silver shield, engraved "BOER WAR / T.W.S". Further engraving to the copper reads "COLENSO / 15.12.99". The second napkin ring (not shown) has the ubiquitous "sawtooth" groove pattern, as formed by Mk II rifling, which was used with both 12 and 15-pdr BL Mk I guns. Note: the napkin rings were made with the bands inverted.








The “crenelated” rather than “sawtooth” profile, along with the number of grooves (18 rather than 12), indicates that this pattern or ridges and troughs was created by travel down the bore of a 15-pdr BL Mk I gun, with Mk III rifling.


A piece of driving-band from a 15-pdr with Mk III rifling (top), compared with other 12 & 15-pdr fragments. Note, in particular, the difference between the crenelated profile of the Mk III rifling and the sawtooth or hook cross-section of its Mk II equivalent.



But, why the strange narrow wedge-shaped ridges?

The answer to this lies in the nature of the twist employed with Mk III rifling. Instead of having a twist along the entire length of the bore (as found with Mk I & Mk II systems), the Mk III design relied on a straight section, 18 in (46 cm) long, which then changed to a clockwise twist for the remaining 53.6 in (136 cm) of the barrel.







The process

The straight section of rifling creates ridges as one would expect (i.e. parallel to the axis of the barrel). Then, due to the force imparted by the transition to a twist system, the rifling pushes the previously raised copper areas to the right, this time on the new angled alignment. The left-hand side of the original ridge becomes flattened and is reduced to ghost lines, while its right-hand edge is retained. Where the copper has been flattened by the bore during the first phase, there is no metal available to fill the rifling grooves in their relative positions on the new alignment. Therefore, the new right-hand edge appears only as an almost imperceptible lip.

This results in the distinctive tapered or wedge-shaped ridges, angled on the left side and straight on the right.





The above diagram attempts to show the two processes that create the distinctive Mk III rifling grooves.

The left-hand image shows the formation of the initial ridge pattern, a straight raised area of copper, aligned on the axis of the barrel.

On the right, the parallel red lines show the adjusted alignment, reflecting the twist of the remaining 75 percent of the bore. The left-hand side of the original ridge becomes flattened and a new faint lip appears on the right. Because the straight section of the bore has already flattened this area, there is not enough copper remaining to form a complete, uninterrupted ridge (as seen with other marks of rifling).


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Excellent research, Neville. Explained very clearly and supported by excellent images.