Hmph.

If you're going to directly rip off the gas turbine engine from my design (it's an aircraft APU unit weighing approximately 17kg), do it properly

A direct drive off a gas turbine won't be any good as the turbine's powerband will be pretty much at the peak rpm of your weapon - i.e. it won't be making much power when you're getting it running initially. Also, the shock of suddenly loading down the engine when you clutch it to the weapon will probably cause a compressor stall and wreck it.

Also, your spinup times you've stated are faster than mine - except you aren't running the engine at its maximum output for most of the duration of the spin-up, so you're clearly being ambitious there. :P