I think it should be detailed and accurate, but if you don't have the time for it on day one release, maybe as updates later on.
Small arms definitely rank lowest on the scale, with direct fire cannons (i.e. tank cannons) come second and then artillery and missiles etc.
Assault rifles, depending on the model should really have about 800 metres maximum with the full rifle round (7.62 mm) and around 600 for the 5.56. Sniper rifles could shoot at around 1,500 metres, machine guns would be the same as the assault rifle but with the heavy round (i.e. 12.7 mm, 14.5 mm) up to about 1,800 metres. Rocket launchers possible about 100 - 200 metres. Pistols would be around 30 metres and sub-machine guns around 30 - 100 metres. But these can all change depending on the specifics of the weapon used and the amount of training the person who has used it and how trained the person is in general. For example an elite unit would be able to fire farther more accurately with less rounds being wasted than regular infantry, and they in turn would be far better than militia.
Cannons are a bit more tricky. Certain rounds fire farther than others because of the projectile. But that could possibly be implemented later if you want to go into real detail. 105 mm cannons (i.e. L7) could have a maximum effective range of around 2,000 metres, whereas 120 mm guns (i.e. L11) would be around 2,500 metres. 100 mm guns would be about 1,600 metres (they were not effective at range, inaccurate) and the 115 mm and 125 mm guns would be about the same as the 120 mm guns.
Artillery would be 15 - 20 km+, not sure how far each model would go.
Missiles would be roughly around 5,000 metres if not farther.