"Rifle" is a bit ill defined here. A rifle is:
a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves ("rifling") cut into the barrel walls. The raised areas of the rifling are called "lands," which make contact with the projectile (for small arms usage, called a bullet), imparting spin around an axis corresponding to the orientation of the weapon.
That would increase flight stability of bullets, but the fins do that on arrows and darts already.
What the Chinese had was at least a repeating crossbow:
The repeating crossbow (Chinese: 連弩; pinyin: Lián Nŭ), also known as the Zhuge crossbow (Chinese: 諸葛弩; pinyin: Zhūgé nǔ, previously romanized Chu-ko-nu) due to the design upgrade contributed by Three Kingdoms-era strategist Zhuge Liang (181—234 AD), is an ancient Chinese crossbow where the separate actions of stringing the bow, placing the bolt and shooting it can be accomplished with a simple one-handed movement while keeping the crossbow stationary. This allows a higher rate of fire than a normal crossbow: there is a top-mounted magazine containing a reservoir of bolts that are fed by gravity, and the mechanism is worked by simply moving a rectangular lever forward and backward.
A video of this in action is here.
Bullet shooting crossbows:
A bullet-shooting crossbow, sometimes referred to as “stone bow,” is a modified version of the classic crossbow. The bow was usually constructed with wood or steel, depending on the preference. It typically utilizes bullets and stones as projectiles instead of the traditional quarrel. […]
Many intended to use the bullet-shooting crossbow as a weapon, but it had its limitations. While the bow worked well on small animals like squirrels and birds, it was not powerful enough to be a reliable weapon in war. The velocity was just too low for the bullets or stone to pierce skin, and therefore it didn’t cause much damage to humans. The bullet-shooting crossbow had the potential to fracture skulls, but as this was the only effective way to take down the enemy, the bullet-shooting crossbow didn't match up with the standard crossbow in wartime.
In addition to the low-velocity shots of the bullet-shooting crossbow, the projectiles were not as effective as the bolt on the standard crossbow. The sharp bolts of the crossbow did more damage than the slow-moving, blunt bullets of the bullet-shooting crossbow. Because the standard bolt does more damage, the bullet-shooting crossbow was reserved for hunting smaller animals. There was no reason to use a bolt-shooting crossbow to hunt squirrels when a smaller, less-powerful weapon could get the job done just as effectively.
Another flaw lies in the material of choice: steel. Steel bows require more energy to return the bow to its original position and therefore deliver less energy to the shot itself. Bows made of wood typically shot much faster than those made of steel.