(answer is based on 100, lonely, 31 IVs)
You can find out these things using a Damage Calculator
In the case you have mentioned, against defensive Pokemon with a resistance... not too much damage.
252+ Atk Choice Band Sheer Force Rampardos Zen Headbutt vs. 252 HP / 252+ Def Aegislash-Shield: 60-71 (18.5 - 21.9%) -- possible 6HKO after Leftovers recovery
252+ Atk Choice Band Sheer Force Rampardos Zen Headbutt vs. 252 HP / 252+ Def Bastiodon: 55-65 (16.9 - 20%) -- possible 5HKO
252+ Atk Choice Band Sheer Force Rampardos Zen Headbutt vs. 252 HP / 4 Def Bronzong: 49-57 (14.4 - 16.8%) -- possible 9HKO after Leftovers recovery
Against Defensive Rotom-W: decent chance of a 2HKO
252+ Atk Choice Band Sheer Force Rampardos Zen Headbutt vs. 248 HP / 252+ Def Rotom-W: 153-181 (50.4 - 59.7%) -- 85.9% chance to 2HKO after Leftovers recovery**
Against Shuckle... meh
252+ Atk Choice Band Sheer Force Rampardos Zen Headbutt vs. 248 HP / 252+ Def Shuckle: 86-102 (35.3 - 41.9%) -- guaranteed 3HKO
Against Heracross... devastating.
252+ Atk Choice Band Sheer Force Rampardos Zen Headbutt vs. 164 HP / 0 Def Mega Heracross: 396-466 (115.7 - 136.2%) -- guaranteed OHKO
I could go on, but there are 720 Pokemon with many different sets. It's probably an OK move to use as long as you don't come up anything that resists Zen Headbutt. But with common weaknesses and a lousy speed stat, you're not going to sweep with Ramparados.
