Sourcing weaponry for customisations can be an uphill struggle: there may not be the appropriate weapon in the correct scale for use, or the weapon would fit the look you are gunning for.
I stumbled upon these 1/6-scale modern weapon sets during my usual online browsing, and decided to give them a go. Fans of modern shooting games would recognise them easily, as would servicemen in the military or gun enthusiasts. The kits themselves come together rather easily, building like actual guns (bolts, receivers, the works), and come with magazines, rounds and all sorts of paraphernalia. What's important though, is the fitment.