971 But Maybe In A Tunnel

Once everyone was suited up and the equipment was double-checked, it was time to decide what sort of round for the Mass Driver they were going to need.

Using a Hypersonic Solid Projectile in a tunnel system was a terrible idea from almost every possible angle. Fortunately, the Mass Driver was not a fixed velocity weapon, and it could be used to propel the rounds at subsonic speeds and drastically reduce the sound generated.

Of course, a subsonic projectile in .22 calibre would do a grand total of nothing to an armoured opponent, but they still had options—namely, the high explosive rounds. 

They would still detonate on impact just fine, but the explosions would defeat the purpose of slowing the rounds down.

So, a moderate approach was chosen. If the rounds were fired at only ten times the speed of sound, the noise would be tolerable, comparable to a standard projectile weapon like the ones that the enemy was likely to be using.