Yeah, the tips of your claw, where bullets shoot out of, are invulnerable.. so you can shoot enemies that are on the rim by sort of gently easing towards them, making that pincer move 'into' them and so shooting them. Go a bit too far, too quickly, and the centre of your claw will hit the enemy though. The brake control (left trigger iirc?) is helpful with this too cos with that you can just sit still and push left/right to shoot the enemies as they come up to you - BUT even with this, depending on the exact alignment of your claw/the web/the enemy, the enemy could spin 'around' your bullet and come down right on you.
It's weird and seems a bit random but you'll get a feel for it with practice, and then it feels good because with some subtle stick nudging you can blast through seemingly impenetrable walls of baddies =)