The technical term for that is a jackshaft. A double reduction will work but has some drawbacks: It will add weight and complexity and takes up valuable space. There is also a potential problem when you have a small pulley close to a larger pulley. The small pulley will have very few teeth fully engaged with the belt (maybe only 4) and those teeth have to transmit all the motor power. this graphic explains it better:



When the blade hits something, there will be a huge amount of force on those few teeth and they may strip off the belt, or they might slip and start to wear down. Either way, the belt will wear out much faster than it should. The way to prevent that is with an idler or 'back-breaker' wheel, which increases the belt wrap around the motor pulley. Its also a good way to adjust the belt tension.

The alternatives to using an idler are to use a wider belt and/or larger pulleys. You can also use a smooth pulley on the weapon shaft to act as a clutch and limit the amount of force on the pulley teeth. I don't really trust this option as its hard to know what the right amount of tension is.

All the above solutions will work but the best or at least most elegant solution is to find a motor with a lower KV and have a single reduction to get a weapon speed of around 3.5K rpm, with a 4:1 reduction ratio and a 7S battery, you will need a motor with around 600 to 650KV.