+1 with all the guy's before me.
If you felt the M16 was to light and didn't feel right, try one of the support weapons: M60, M249, MK43, MK46. Most players find these weapons to heavy to use all day long. But a few exceptions use them as their main weapon with great success on the battlefield. An upgraded MK46 will dominate a whole area of the battlefield if upgraded and used properly.
One of my friends is a legend on the battlefield because he uses the an upgraded MK46 to spray the opposing team into submission
He fires 3000-6000 bb's in a game but enjoys every minute
Support weapons can now be purchased for 400-500$ plus upgrades to handle the rps (rounds per seconds).
Take care, 5kull