Another thing that can contribute to inaccuracy is the stiffness of the barrel, and how consistently it vibrates while having a bullet travel through it. Bullets of different weight will change how the barrel vibrates, and since the bullet is turning while traveling an inconsistency in it's weight distribution could alter the vibration of the barrel, ultimately changing the point of impact. The relationship between the weapons sighting apparatus and the barrel can, and does shift over time, resulting in inconsistency. A trigger's over travel can alter the position of the rifle before the bullet exists the barrel. The amount of time it takes for the bullet to exit the barrel after the trigger breaks, if not consistent can lead to inaccuracy.
Generally weapons are pretty consistent. Ammunition cannot always be perfectly consistent, but their inconsistencies are usually negligible for the common shooter.
By far the most inconsistent element of shooting is the shooter. A person physically cannot hold something exactly the same way every time they pick it up. Granted those that practice lots can get pretty close, but EXACTLY the same is impossible. Muscles fatigue over time and differing resistance to recoil can affect accuracy. Pretty much every interaction you have with your weapon can adversely affect accuracy.
This is why I love shooting, there is so much going on from physics to chemistry and biology. There's always something to learn, to understand, and to conquer in the quest for better performance.
So I suppose an inaccurate gun is an inconsistent gun.
Imagine what I would do, if I could do all I can.