The repeating heavy crossbow was an actual weapon used since the 4th century BC becoming the standard military weapon as early as the 2nd century BC. It is also one of the longest lived mechanical weapons ever devised.

The repeating crossbow could hold 10 rounds and fire them all withing 15 seconds, thus the fifth ed D&D equivalent allows for up to 4 shots per round as training improved use.

The existing rules for a fighter's increase in attacks per round, including the use of an action surge, allow the weapon to be fired once per round at 1st level. With the Action Surge at 2nd level, the weapon can occasionally be fired twice. At 5th, 11th, and 20th level the numbers increase to 2/3, 3/4, and 4/5 shots per round respectively.

The weapon had a shorter range than a regular crossbow, but since D&D improperly gives the crossbow a shorter range than a longbow (historically, crossbows had similar, if not greater range than longbows), I give it the same range as the regular crossbow.

Exchanging cartridges was quick and easy taking an action to complete.

The weapon was extremely easy to make and easy to use according to historical records but I gave it a 10gp increase in cost and a 2lb increase in weight for game balance.

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The multishot heavy crossbow was an actual weapon used since the 3rd century AD.

The multishot heavy crossbow could fire 3 bolts per attack and were reported to fire farther and faster than the repeating crossbows of the time. Since D&D improperly gives the crossbow a shorter range than a longbow (historically, crossbows had similar, if not greater range than longbows), I give it the same range as the regular crossbow.

Reloading was fast and easy taking one action to complete.

The weapon was extremely easy to make and easy to use according to historical records but I doubled it's cost and gave it a 2lb increase in weight for game balance.