The 'thing' on the front is a reactive flash suppressor, normally found on the M82 and 90 series. The Americans call it a muzzle brake.

It's basically there to ensure even distribution of expelled gases to ensure stability and to minimise flash and smoke. You can date the rifle by the shape of the brake - they were originally double round cross sections, now they are double rectangles. Snipers use suppressors on the Barret series to enhance stability and to minimise the chances of being spotted. It's not the same as a silencer.

Not sure if you are trying for a realistic model so no crits to offer really, apart from the foregrip on most Barrets is more like a 6 sided cylinder than a square.