Yeah some thing like that, And it's not that Big.
With that I would put in a M249 Gear-Box and cut off some of the top stick in a Hop-up to a P-90, then put the P-90 mag on top, And make the Mag look like the Top of the weapon. But the battery in the butt Stock, Pill out for the Barrel, And Make a Trigger for it.
And Maybe Stick in a Tracer unit, So at night the BBS will glow, so it looks like a laser blast!