There's something really weird about weapon damage. It's like you're not allowed to one-shot some mobs with a primary weapon.
I'm a L8 Titan with a 62 attack Jigoku scout rifle (the highest impact of them all).
I can shoot a level 1 vandal for 199, leaving it with about 2 pixels of health.
I can shoot a level 4 vandal for 243, leaving it with about 2 pixels of health.
I can shoot a level 5 vandal for 243, leaving it with about 2 pixels of health.
I can shoot a level 9 vandal for 247, leaving it with about 5 pixels of health.
There's a pattern there, but it's not one that makes any damn sense.