Point singularity projectors... back in the old days of andromeda, one of the fancier weapons was a psp. Basically, a small black hole launched towards a hostile. Of course, it keeps going... and going.. and going.. etc etc :p (see above ME2 link)
But really, it working similar to anti-ballistics weapons today is probably more likely. It could also be a laser pulse that destroys the railgun ammo... lasers would require slightly less computational tracking... Would explain the short range though (as distance goes up, area goes up at the square...) so too large an area and all the calculations might be hard.
I digress. Basically, it shoots down railgun ammo. do we need more technobabble?