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405 SPOTTER (10mm XM75)

Frankford Arsenal started the development of a new shoulder-fired 90mm recoilless rifle in 1957, the PAT (Platoon Anti-Tank, which was to use a spotting pistol incorporated into the forward grip. Initial testing was done with a scaled down .50 Cal spotter-tracer in a modified .30 Service case (.30-06) but results were not encouraging and a .405 (10mm) bullet was designed. In early 1958 Frankford Arsenal assigned the designation FAT72 to a .405 cal spotter cartridge. These were loaded initially with an inert TAT-82 spotter tracer bullet with no markings for ballistic testing. An additional lot was loaded with yellow/red tips.

                              

   High Pressure Test Load using the FAT73 brass case    

  

With the redesign of the FAT73 case in September 1959 a high-low pressure filter was added to the case to reduce pressure and velocity deviation. A groove was added to the case to secure the filter but fired cases were examined and it was found that the groove was not able to prevent the filter from being blown out. To correct the problem, a metal ring was added to the groove. The bullet was designated FAT82E2 and had a boat-tailed base, grey plastic rotating band and an open GM jacket.