[AmesJainchill]

The description of the weapons points out, the primary disadvantage of EM guns is their high power consumption. You have to replace a C-cell (about the size of a pistol magazine) for every magazine of actual projectiles you fire from an EM gun. The subsonic option lets you cut that to one power cell for every 4 mags, but you cut your damage and range by half, which for a 4mm projectile is a big problem.

I assume the option for subsonic rounds is if you are low on power cells but not ammunition, or if you want to use a suppressor or just make less noise without a suppressor or just not waste the power on a shot that doesn't need it, say with the Sniper Railgun, 7mm, which does 6dx2 (roll 6 six-sided dice for damage, double the result) normally, but with a magazine of 30 shots you might want to just use the low power setting (half damage and range) for a relatively unarmored person within the lower range. You'd need the 12d6 high setting to disable vehicles or heavily armored soldiers or heavy equipment or whatever.

12d6 is overkill on anything but the heaviest armored people, really.