Calling All Hunters and Gun Experts

If anyone out there knows the make, model, and modern-day equivalent to the gun that shot JFK, please leave me a nice comment telling me this.

I need a murder weapon for my mystery I'm writing, and this gun is the only thing powerful enough to send a 200 lbs. man flying 2 feet backwards off a 10 foot high stage. At least in my little fiction.

The only thing I ask is that the gun needs to be a modern-day hunting rifle, preferably one that can have a silencer and a sniper scope put on it rather easily.