Night Shade and Seismic Toss always deal damage equal to the user's Level if it hits, so a Level 100 Pokémon would always do 100 Damage, regardless of Offensive or Defensive stats. :P
Dragon Rage always does 40 Damage if it hits, even if the user is level 1. :P
Sonic Boom always deals 20 Damage if it hits. :P
Gible Dragon Rage vs. 252 HP Shuckle: 40-40 (16.3 - 16.3%) -- guaranteed 7HKO
Magnemite Sonic Boom vs. 252 HP Shuckle: 20-20 (8.1 - 8.1%) -- possibly the worst move ever
When did you use Dragon Rage against Shuckle? I am unsure why it did less damage. Could you explain the battle conditions a bit more? :P
Edit: If you used Sonic Boom, it would have done about 7% damage to Shuckle. Are you certain you didn't use Sonic Boom? :P
Hope I Helped!