Fusion guns are Super-Destructive, which is something like double damage to Armour. The idea is that high energy weapons, like the powerguns, expend all their energy when they strike the target. They penetrate by brute force, doing so much damage that some goes through. All that damage goes somewhere, in this case into the armour itself. This rule works best when used with the Armour Location rules, as it would only damage armour along one facing. The rule was playtested in Hammer's Slammers playtesting,
In Traveller vehicle combat, a Miss is a Miss. Not a bounce off armour, but a Miss. Armour doesn't affect To Hit.
If a vehicle has no armour, then that excess energy is effectively wasted. It could go in Hull, but the rule isn't written that way. A high-energy strike on an unarmoured vehicle would do a great deal of damage to it regardless.