My customized Transformers Generations: Fall Of Cybertron Shockwave. Custom metallic purple and silver repaint.
Custom 'hose' running from his back to the fusion cannon, which is clipped onto his left forearm.
His 'hose' is made from an old USB cable, which has been plugged into the hole in his back through a small rubber gasket and held in place by friction.
I made a small hole in the laser cannon with a tiny screwdriver, towards the top, and inserted the cable into it, and is also held in place by friction.
His laser gun / cannon is clipped into his forearm. If his left hand is not flipped out, there is a slot between his 'wrist' and hand like this  .
There is a small tab inside his cannon behind the handle he would usually hold, like this || O .
The tab inside the cannon clips into the slot in his forearm.
It's a tight fit, but it will hold very well.
Any other questions, please feel free to ask Photo and idea (C) gamera68 AKA bumble_bee_68.