The development behind firearms accelerated during the 19th and 20th centuries. Breech-loading became more or less a universal standard for the reloading of most hand-held firearms and continues to be so with some notable exceptions (such as mortars). Instead of loading individual rounds into weapons, magazines holding multiple munitions were adopted—these aided rapid reloading. Automatic and semi-automatic firing mechanisms meant that a single soldier could fire many more rounds in a minute than a vintage weapon could fire over the course of a battle. Polymers and alloys in firearm construction made weaponry progressively lighter and thus easier to deploy. Ammunition changed over the centuries from simple metallic ball-shaped projectiles that rattled down the barrel to bullets and cartridges manufactured to high precision. Especially in the past century has particular attention been devoted to accuracy and sighting to make firearms altogether far more accurate than ever before. More than any single factor though, firearms have proliferated due to the advent of mass production—enabling arms manufacturers to produce large quantities of weaponry to a consistent standard.