I know you have been all waiting for this so here's the answer: from wiki
"An Armstrong gun was a uniquely designed type of rifled breech-loading field and heavy gun designed by Sir William Armstrong and manufactured in England beginning in 1855 by the Elswick Ordnance Company and the Royal Arsenal at Woolwich. Such guns involved a built-up gun construction system of a wrought-iron (later of mild steel) tube surrounded by multiple wrought-iron strengthening coils shrunk over the inner tube to keep it under compression."
It was made from a 3 pounder up to a 110 pounder, had a rifled barrel and used a projectile instead of a ball.