This book series I've read quite a while ago (I think it was Crimson Worlds, but it may have been a different series as it was a long time ago and I've read so many) that the FACs of one faction was a small ship with a dozen crew that went stupidly fast (only matched in speed by courier ships which are just a small crew section and giant engines) and was equipped with only a big spinal railgun. Despite the tonnage being significantly higher than its equal in Aurora, because a "FAC" is a step just below the smallest warship (corvette) but still a military vessel, and automation in the series, this ship type equates to a FAC.