投射机（Ballista）或称扭力弩砲，是以绞绳的扭力（而非「弓身」的弹力）来发射物体的大型冷兵器砲。此器与弩、床弩依靠弓身的弹力来发射物体的运作模式不同。投射机最早可追溯至古希腊时代。古罗马军队使用的投射机有分大型与小型，小型的投射器又称作蝎砲（Scorpius）。另外也有单兵使用的手持投射机（扭力弩）。

古罗马投射机。

图拉真柱上的一张古罗马投射机（蝎砲）。

古罗马投射机的箭头。

一张九英吋长的小型古罗马投射机。注意: 图上的扭力绳比历史上的实物还短。

一个四世纪的古罗马投射机的金属原件。

For the Roman general, see Balista. For the Italian actor, see Gigi Ballista. For the sniper rifle, see FN Ballista

The ballista (Latin, from Greek βαλλίστρα ballistra and that from βάλλω ballō, "throw"), plural ballistae, was an ancient missile weapon that launched a large projectile at a distant target.

The earliest mention of ballista in literature occurs in the Bible, as invented and used under the reign of King Uzziah: II Chronicles 26:15: "Moreover, Uzziah prepared for all the army shields, spears, helmets, body armor, bows and sling stones. 15 In Jerusalem he made engines of war invented by skillful men to be on the towers and on the corners for the purpose of shooting arrows and great stones. Hence his fame spread afar, for he was marvelously helped until he was strong" "The Hebrew word in question, באלליסתארי״מ, when translated to Latin is ballistarium which occurs twice in the Greek version of his (Ezekiel) prophetic description of the dreadful siege which Jerusalem was shortly to undergo. באלליסתארי״מ, corresponding to the Latin word ballistarium, means a platform for artillery. The Hebrew text, however, gives karim which means battering rams, so that person that translated Ezekiel's work into Greek, at the time when artillery was very common was led into making a simple, but understandable, mistake. Incidentally, those responsible for the Authorized and Revised Versions of the English Bible avoided an error. Thus, the evidence for early artillery in biblical texts is misleading and due to human failings in their composition and transmission".