All modern armies owe something to the Legions of Ancient Rome. A fascinating book, The Last Legionary by Paul Elliott, describes, as its subtitle suggests, Life as a Roman Soldier in Britain, AD 400.
The book combines, in the style of Christopher Matthew’s A Storm of Spears (on the Greek hoplite at war; only $1.26 at that link; previously mentioned here in comments and here), the disciplines of history, material archaeology, and “experiential archaeology” as practiced by reenactors. Where The Last Legionary is different is that its facts about the Roman military’s last years in Roman Britannia are woven into the story of an simple soldier, we guess you could say an ordinary Gaius. Gaius was born in 362 to a Roman legionary, Maritius, and his wife, and on reaching his majority was compelled to join up under the edict of Diocletian, which committed sons to their fathers’ professions. Some youths dodged the draft by cutting their thumbs off, which was discouraged initially by burning the draft dodgers and later by drafting them anyway.
Gaius was no draft dodger, and accepted his fate. He swore an oath (to Christ and the Emperor) to serve, and if need be, die for the Roman Empire. Training was harsh and hardening, including formation drill, fast marches, position and fortification construction, and plenty of physical training.
Of most interest to our readers is probably the weaponry on which Gaius was expected to gain proficiency. While the Roman Legion of Caesar’s day fought primarily close-in with spear and short sword, by the fourth century projectiles were a major part of combat. To be a properly cross-trained legionary, Gaius would have to learn to master the sling, the recurve bow, the plumbata dart, three kinds of javelin, the crossbow ,and the barbarians’ own throwing axe, as well as the classical sword, shield and spear of centuries before. Indeed, missile weapons training usually began before close-in weapons training.
Late Roman Missile Weapons
The sling was a leather or woven cup with a cord proceeding from two corners. One cord is looped around the index finger, that’s the standing end of the sling; the other is tied in a knot, which is the running end, and the slinger releases it to launch the projectile — a stone, or a lead ball — but accuracy is hard to achieve, the author has learned. Roman sources suggest a single whip round, and setting the practice targets at — wait for it — 180 meters, same as for bows. Elliott has been unable to achieve this range, with the regular sling or using one with the cords proceeding form a stick.
The recurve bow came to the Roman army from encounters with Eastern enemies so armed. Mostly these were the tribes of the East; for centuries the Romans had trouble with Scythians and Parthians, among others.

The Romans adopted the recurve bow after seeing its effect first hand, and while it only bought them parity in the East, in the West it gave them technological superiority to the “self-bow” of the Gauls and Germans.
Often the ends or “ears” of the bow was strengthened with bone laths, and the body of the bow was carefully covered with leather to protect it from moisture. Wet conditions could ruin a recurve bow, as could misuse. Leaving the bow stringed and ready for action ruins the springiness of the bow and reduces its power. Unlike the sling, specialist craftsmen were needed to make these complex weapons.
The bowstring was drawn differently in units raised and trained in the eastern and western units. Western-trained archers shot using the fingers of their strong hand; Eastern-trained archers used a thumb ring. While archers could fire at individual targets, they were often used in volley fire.
The plumbata was a recent (~4th Century) addition to the Roman grunt’s panoply. It was a lead-weighted dart, shorter than an arrow, that could be thrown by hand or launched — as far as 100 m! — with a sling or throwing stick.

Reconstructed plumbatae. Source.
Testa by the historical research group, Comitatus, have found that an underhand throw was by far the best method.The plumbatae can reach an impressive distance, easily exceeding 60 m, and come down vertically directly onto the heads and shoulders of the enemy.
This is a different re-enactor group throwing plumbatae. From Roman-Artifacts.com.

Another Roman name for the plumbata was the “Barb of Mars.”

Surviving plumbata head. Source.
Romans used several types of javelins, known by the names pilum, spiculum, and verutum, but while these were nominally throwing weapons, they were hard to throw accurately or any distance.
Two weapons of secondary importance were the throwing axe, adopted from some of the northern Germanic tribes, and the crossbow, which was probably developed by scaling down a siege engine, but was rather new at the time. (Elliott cites sources that make it clear that the late Roman Empire deployed this weapon, which he points out that most people associate with medieval warfare. There was more continuity between antiquity and modernity than “dark ages” historiography suggests).
Late Roman Close Combat Weapons
The two basic combat weapons of antiquity were the sword and the spear. Technology had not stood still, and the soldier who fought blue-painted Britons in Roman Britain wasn’t armed quite like his ancestor in Caesar’s legions had been.
The sword was the spatha, a longer (~700mm) sword than the classic gladius of Caesar’s age. It was originally a weapon for cavalry.
Spatha were not crude mass-produced weapons, they were carefully wrought swords, often with pattern-welded blades. These blades were formed from several iron bars, all of different carbon content, that would twisted into a screw shape and then hammered and folded repeatedly. To this strong, yet flexible, core, hardened steel cutting edges were welded. The blades are strong and beautiful, with long straight sides and sharp points. The hilts and pommels were crafted from wood, horn or bone – all organic materials. In earlier centuries, the legionary sword hung on the soldier’s right side, but in the fourth century, soldiers wore their swords on the left, traditionally the preserve of centurions and senior officers.
In man to man combat the sword was used to stab into the body of a foe, but when engaging a shielded target the long spatha could be used to reach over the Shield to strike the head or neck, the shoulders, the sword arm, or the left leg….
If the spatha was the 400 AD legionary’s offensive weapon, his tactical defensive weapon was the spear. Spears had seen a lot less technological change in the preceding 400 years, but that’s for the best of reasons: they were quite well evolved already. A single spearman on the battlefield would have been vulnerable to being flanked and defeated by more agile foes, but no army — certainly not the Romans! — fights as individuals. Attacking a unit of spear-armed Romans was a mortal-consequences game of Slap The Porcupine. Wise enemies didn’t try, and unwise ones died or wised up PDQ.
The Roman shield, on the other hand, had changed since Caesar’s day. Caesar’s legions carried a rectangular shield, that in overhead plan view had an arc to it. The late Empire infantryman had a round shield, which worked better with the long spatha.
The infantryman of 400 AD had a great many weapons to master, along with all the other soldier skills of the day. And we enjoyed learning about his training, combat, and life in general, in The Last Legionary.