>>356Jk Rowling mentioned before that guns are actually more effective at killing, and that a muggle assassin with a gun could have easily killed volemort
The advantage with Avada Kadavara is that: you don't need bullets, just hatred. You can hit any part of their body, and it is instant death.
The disadvantages are that you typically have to announce it, unless casting silently, you have to use a wand, unless you could cast it wandless, and while it does arc out at pretty fast speeds, it is technically dodgeable.
Personally it is one of my favorite spells, along with Crucio, and Sectumsempra.
If you don't know, sectumsempra