Because of WW3 happening Misaka's mom tries to pull her daughter out of the city, but Crowley can't have that and puts a hit out on her. Accelerator decides he wants to try and save her, so Crowley turns off his collar so he can't use his powers, Accel is like 'so fucking what' and pulls up with a pistol and blasts the gang members, without his powers and while needing a cane to walk.
It doesn't end there though cause Touma also made his own attempt to save Misaka's mom and because the structure they were in was dark Accelerator mistakes Touma as a gang member and started unloading on him while he's carrying the mom.
Accelerator had more of a chance of killing Touma with a gun than his powers.