Lithium Ferrite batteries run at 9.9v and come in a multitude of shapes and sizes. Adds a bit more juice to your gun due to the voltage, should be handled with care and never fully discharged.
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Li-Fe Batteries
Vapex 9.9V 1600mAh LiFePO4 Battery (Mini)
Useful compact battery, equivalent in size to standard 8.4V 2/3A mini batteries. 40A discharge capability. 103.5 x 34.6 x 17.5mm weight 118g.