The difference between the magic bullets and Mikoto's coins or Accelerators I-beams is in what they are comprised of.

Mikoto or Accelerator alter the reality of the forces around the coin or beams, but not the actual objects themselves. The dropped car on the other hand, was spontaneously created and is a construct of altered reality. That's why Touma would be able to block it. At the same time though, his arm was chopped off. That particular one doesn't follow the same reasoning.

Well....magic aside (I'm starting to believe my assumptions that ESP and magic are accomplished in a equivalent way are incorrect), I don't think Touma can stop any projectiles powered by ESP. He seems to know this as well, and avoids them accordingly.

He could certainly cancel out the electromagnetic forces that generate the momentum to railgun a coin, but once Mikoto is done with that, the momentum of the coin would be a part of the true reality. Her power doesn't encompass momentum the same way Accelerator's does, so he shouldn't be able to cancel that out. It isn't like he can undo what has already been done, a damaged building is a still a damaged building.

There is a different set of rules with Accelerator. If his power is accomplished instantaneously, it would be the same as Mikoto's railgun, Touma can't block it. However, if Accelerator governs the object after he's tossed it, Touma would be able to stop it. But Accelerator described his power as the instantaneous change in vectors and momentum. So Touma can't stop what has already been tossed, just prevent Accelerator from doing anything. Touma probably knows this, and dodges accordingly. Accelerator's power covers his entire skin, so if Touma grabs onto him, Accelerator would become a powerless, normal person.


While I think that Touma's arm reintroduces uncertainty to the "certain" altered reality of Espers, how his arm accomplishes the negation of magic must be something different. He has not had the same consistent experiences with cancelling magic. There is almost an area of effect around his arm, rather than point-to-point. He seemed most inconsistent with Kanzaki. Her powers were a mix of physical attacks and magic-powered tricks (the tatami mats against Index). Touma was weak to a lot of that. The same goes for the feathers, he cancelled out what created them, but unlike the heat that Stiyl's fire creates, Touma couldn't stop the feather from wiping out his memories when it touched his head.

Perhaps his power against magic is a more conscious effort.