Just to clarify, what do you mean by burst damage? The term usually reffers to a short durration of high damage. If you can sustain it for long periods of time then it's not really burst damage. If you're just talking about the damage of your normal attacks then you shouldn't worry about that on rifle and mech guns, they always have weak normals. It's their PAs and skills that make them.

Like isCasted said, Weak bullet is very important, but you'll have to sub or main ranger for that, so keep that in mind.