Well if you want to train ranged fast, the cannon isn't a bad idea, but using it at crabs, that is a bad idea.
1. They have low hp, so yes you do get to kill them one hit, but for crabs you need to "bring them to life" first, meaning you still need to walk around thus not being so fast.
2. I would recommend going to the black demons in the members dungeon and set it up in a safe spot there (as well as yourself) and use a d hally or something else so you can train melee as well as ranged, of course dont use the hally if you're a range pure or something along those lines. Plus! black demons will give you wayy better drops than the crabs...
In taverley dungeon theres black dragons across the fire battlestaff crafting place, there is a ladder there i believe. set up the cannon and hide behind the ladder.
Then do the fire giants, or hell go to the mossies by ardy. That's always a good spot. I trained there from probably 50 to 65 range then moved onto the fires and other random monsters up to the lvl 74 range I have right now.