After reading reports of frequent fires in Sherman tanks coming from the British, the Americans decided to investigate. Separate trials were done on tanks cleared of ammunition, and frequent fires did not result from penetrations. However, the story was completely different when the opposite case was tested: the tank had a full loadout of ammo, but no traces of fuel or oil.
90% of penetrations of the fighting compartment and the turret caused a fire! To make things worse, CO2 fire extinguishers proved ineffective, and only large quantities of water could fight this fire.
To test how well water helps against these fires, a rack surrounded by water was tested, but the first penetration tore the entire rack apart due to hydraulic action. A solution was to use concentric cells, with the space between them filled with water, with some air left on top. This proved to protect the ammunition better, with one one cell bursting when hit. A new layout for ammunition was also drafted, which would not only protect it with water, but keep it out of harm's way below the track line.
Trials of the new ammo racks installed in real Sherman hulls proved promising. Out of 14 ammunition cells punctured by 37 and 75 mm shells only two caught fire. The reduction of the vulnerable area also went a long way to keep the tank safe. In addition, despite fears that this kind of modernization would decrease the amount of available ammunition, the total capacity increased to 150 rounds. However, the ready rack was reduced to only 6.
It was concluded that, short of armouring a tank to the point where it cannot be penetrated at all, this is the best available method of protecting from ammunition fires.