I should look it up before typing, but I think the band shrinking process was replaced the autofrettage process.
Not that we are anticipating the construction of a "built-up" gun at this time, we have scheduled one to be built in the two year period beginning in autumn of 2013. It will be the next and final gun in our "Magnificent Seacoast Gun Series", the 8", 150 Pdr. Armstrong Gun, a seacoast rifle of the finest quality made in England for the Confederate States of America. Two arrived in 1864 and were captured in 1865 by Federal troops at the Battle of Fort Fisher, North Carolina and later at Fort Caswell nearby.
Anyway, DD brings up an interesting process which is described thusly by J. W. Ryan, Royal Military College of Science, Shrivenham, UK: "When autofrettage is used for strengthening cannon barrels, the barrel is bored to a slightly undersized inside diameter, and then a slightly oversized die is pushed through the barrel. The amount of initial underbore and size of the die are calculated to strain the material past its
elastic limit into plastic deformation, sufficiently far that the final strained diameter is the final desired bore."
An interesting reference to classic, built up guns is also obtained from Ryan: "The effect is that the inner layers of the metal are put under compression by the outer layers in much the same way as though an outer layer of metal had been shrunk on as with a
built-up gun." How about a little terminology here: The
"Bands" that DD mentions are the way CSN Commander Brooke described the 2" thick and approx. 6" wide wrought iron reinforcing bands that were used in 1850s, 60s and 70s reinforced cast iron tube guns.
"Sleeves" are much more modern and this term is used for the 1880s and 1890s practice of breech or muzzle insertion of 8" rifled tubes into 10" Rodman Guns AND today this term is used for muzzle insertion of rifled sleeves into oversize cast iron bores on 10 Pdr. Parrotts or 3" Ordnance Rifles.
"Liners", by contrast is the term used for the steel tube which is located within the cast iron artillery tube flask and has the molten iron poured around it. Built-up guns like the 8" Armstrong are said to be composed of
"Tubes", generally the innermost cast iron or cast steel structures and
"Hoops", the middle layers of machined, heat shrunk wrought iron or steel structures.
"Jackets" are outer layers of reinforcment heat shrunk into position and
"Rings" refer to mechanically fitted, but usually NOT heat shrunk, circular, applied structures which have trunnions forge-welded to them or machined from a solid billet as we do.
Thanks to all for the nice comments on our Rifled Cannon Blanks. We sure hope they save you time and also money. After GOW described that process of making a modern repro built-up gun, I thought, "Good golly, who would sign up for all that?? Serious competitors, that's who! Real serious. Our idea and future product is just another way of creating an accurate rifle with historical profile possibilities without some of the work and most of the complexity.
By the way, we have pictures of one little item that you or your machinist will have to make if you go the Mil-Surp route to make your cannon. To make this applied "Trunnion Ring", we started with a 35 pound chunk of 4150 steel plate 2" thick, pre-hardened to 30 Rockwell on the "C" scale.
Tracy
The ring after about 30 minutes on the band-saw and 40 minutes on the mill to drill centers for lathe operations.
After turning the trunnion diameters and then drilling and boring out the 5.25" tapered hole for the cannon tube, the Ring goes to the mill and rotary table for exterior profiling. Protrusions are included for the front sight mass, the indexing masses which keep the trunnions horizontal and the square rimbase radii, 4X.
The trunnion ring applied to the1/6 scale 7" Brooke Seacoast and Navy Rifle.
Poll