The guns installed at Stony Batter were 9.2-inch (the shells they fired were 9.2" in diameter-- 233.7mm).
A gun weighed 110-120 tonnes. It was heavier if its turret had 2-inch steel armour-plating.
The barrels weighed 28 tonnes; they were 442.75" (11.25 metres) long, and 34.75" (882.7mm) in diameter at the breach end.
The guns were mounted in concrete pits 40 feet (12.19 metres) in diameter.
Three guns were ordered for Stony Batter, but only two were installed. The order for the third was cancelled after the war ended.
A 9.2-inch gun could fire two shells a minute once the crew were trained and if its electricity and hydraulics were working. They could be operated manually; then the maximum rate of fire was a shell every two and half minutes.
9.2-inch guns were used for over a century in most countries of the British Empire where there was a large and important port to be protected.
They were the biggest guns ever used by the New Zealand Army or Navy. The next most powerful in New Zealand were the 6-inch guns at Fort Dorset in Wellington (see the last photo on this page).
Unfortunately, the New Zealand Government destroyed the ones at Stony Batter in the 1960s, even though Waiheke locals asked for them to be kept. They were cut up for scrap, which was then sold to the Japanese (very ironic, considering that their main purpose was to repel any Japanese attempt to attack New Zealand at Auckland).
Photographs of Fort Stony Batter taken during the war are rare because it was top secret. Workers and soldiers, strictly, should not have taken any photos at all. And film was in short supply in wartime.
These pictures show the construction-site, the crusher that was set up to crush local stone for making all the concrete used in the project, a tunnel entrance and one of the gun-pits under construction, a gun being lifted into position, and other final stages culminating in a completed installation--ready to hurl high-explosive, 172kg shells up to 32 kilometres.