Out of the box, the gun is a bit out of balance, not very ergonomic. The drum mag is a bit wobbly and thus not a good part to hold while you shoot. Therefore you will place your hand between triggerguard and mag, which makes it front heavy.
The bananamag installed on the photo solves the problem, it doesnt wobble at all and it lighter than the drum mag, giving proper balance.
I've had it for a year and atm, its one of my favourites(I have a rather large collection), small and yet powerful. Goes well with my WW2 German loadouts.
You can fit a 9.6v battery, no need for a custom one. although its a tight fit.