The next problem encountered by Kilbourn was that the gun cradle, the metal block on which the gun sat, had to be shortened to allow the gun to fit into the Firefly, and thus the gun itself was not very stable. Kilbourn had a new barrel designed for the 17-pounder that had a longer untapered section at the base, which helped solve the stability problem. A new mantlet was designed to house this gun and accept the modified cradle. The Firefly had no armour or mobility advantages over the normal Sherman tank beyond the additional 13 mm of protection added to its mantlet. The modifications were extensive enough that 17-pounders intended for the Firefly had to be factory-built specifically for it.