Onager
There is little evidence for the use of these before the C4 AD, but they are more simple in design and construction than most of what had gone before. It is suggested that the onager comes to the fore when the knowledge and skills required to make the sophisticated catapults of earlier centuries is lost. We are heading into the Dark Age by this time.Roman advances in the design, mobility, and firepower of artillery produced the largest, longest-ranged, and most rapid-firing artillery pieces of the ancient world. Roman catapults were much larger than the old Greek models and were powered by torsion devices and springs made of sinew kept supple when stored in special canisters of oil. As Josephus recorded in his account of the siege of Jerusalem, the largest of these artillery pieces, the onager, (called the "wild ass" because of its kick), could hurl a 100 pound stone over 400 yards. Vegetius noted that each legion had 10 onagri, one per cohort, organic to its organization. Smaller versions of these machines, such as the scorpion and ballista, were compact enough to be transported by horse or mule. These machines could fire a 7-10 pound stone over 300 yards. Caesar required that each legion carry 30 of these small machines, giving the legion a mobile, organic artillery capability. Smaller machines fired iron-tipped bolts. Designed much like the later crossbow but mounted on small platforms or legs, these machines, which required a two man crew, could be used as rapid-fire field guns against enemy formations. They fired a 26-inch bolt over a range of almost 300 yards. Larger versions mounted on a wheeled frame were called carroballistae and required a 10-man crew. These machines could fire perhaps three to four bolts a minute and they were used to lay down a barrage of fire against enemy troop concentrations. They were the world's first rapid-fire field artillery guns.