"Big Gustav" (as named by Krupp, but renamed "Big Dora" by the artillerymen)
weighed over 1000t (1400t including carriage). While a magnificent piece
of engineering, it was operationally a dismal failure. It took 60 trains
to move the gun and associated support equipment/personnel, then a week to
assemble. It built at a cost of 10 million Reichsmarks, originally to
bombard the most heavily defended sections of the Maginot Line, but wasn't
ready in time. It could fire both HE and AP shells, and an AP weighed
about 7000kg. It had an effective range of 47,000m.
Only 53 rounds were fired during the siege at Sevestapol, mostly HE with a
few AP being aimed at concrete bunkers. Only 1 shell in 5 actually reached
the Russians, although one AP shell plunged into the bay through 10m of
water, then through 10m of concrete and into a Russian ammunition bunker.
There was to have been another weighing over 1500t (2000t including
carriage). It was to have been self-propelled using 4 U-Boat engines.
There is a book called "German Artillery of World War II" by Ian Hogg
(Greenhill, 1997) which has details of the ammunition, propelling charges
and primers.
Cheers
David