The guns from Eraser - From IMDb

There are numerous physics/engineering problems with the EM guns. We are told that they fire "aluminum rounds" at "nearly the speed of light". "Rounds" suggests projectiles of at least several milligrams rather than dust-sized. Then the shooter would certainly be knocked off his feet by recoil, which would accelerate him by over 6 mph per milligram of projectile fired. The power source would require about 40 gigawatts per milligram of projectile launched per second; and all this energy would then be released as heat, which would equal the explosive power of about 10 tons of TNT for each milligram of projectile. Further, great damage would be done behind the target: since no material is strong enough to slow a projectile appreciably from such speeds over a short distance, much of the projectile and debris would push through with practically undiminished speed. The actual physics shows that being struck with one of these ammunition rounds would send the victim into orbit around the earth. Also the recoil from the weapon would tear the arms from the torso cleanly.