Come up with a rail gun capable of downing a deer, that's as light as a normal rifle and you'll be onto a winner.
The problem with percussion weapons is that gunsmiths and ballistics scientists reckon that 6,900 Feet per Second is about the maximum velocity attainable, due to the laws of physics. The current most powerful rifle is the Styer Manlicher M2000 anti-attack helicoptor rifle at 6,800 Feet per Second.
A rail gun, however, isn't limited by the laws of physics regarding explosive gas expansion in a gun barrel, so much higher velocities are possible.... which is why the various millitaries show such a great interest.