The bungalow was built for the commanding officer of Fort Nelson during WWII
A World War II Sexton self-propelled gun was used to demolish a building at a Victorian fort near Portsmouth.
The blast began the redevelopment of Fort Nelson, on Portsdown Hill above the city, home to one of the world's finest collections of artillery.
A 1930s bungalow at the front of the museum was ceremonially "blown up" to mark the start of the £3.5m project.
The work will include recreating a "hospital for soldiers", similar to one that existed at the fort in the past.
A new visitor centre will be built on the site of the demolished bungalow and education facilities will also be created along with a gallery.
Fort Nelson was built in 1860 as part of a chain of fortifications forming the city's protective barrier against any French invasion.
During WWII it was used to distribute anti-aircraft ammunition and the bungalow was built for the commanding officer of the fort during that time.
It opened as a museum in 1995, housing the Royal Armouries collection of arms and armour, with more than 350 big guns and cannon on display.
Many of the weapons are now corroding but the work should improve conditions for the collection, which is constantly growing.
The redevelopment will be partly funded by a £2m Lottery grant.