Flintlock guns required the user to pour gunpowder into the barrel of the weapon, stamp it down, add a bullet, then carefully pick the whole thing up and fire it. By the time all that finished, soldiers could be chased down and stabbed with a knife.
Breach-loading firearms opened the rifle near the back. They allow soldiers to insert the gunpowder and bullet at the end, a much faster process. However, early rifles were oftentimes too weak to support the subsequent explosion and would blow up in the shooters face.
Despite that, Scotsman Patrick Ferguson invented the first breach-loaded weapon used in the Civil War in service to the King of England. Ferguson’s guns worked reasonably well but, after the war, Americans captured and killed him due to reports of his incivility to US rebel soldiers.
Breach loading rifles were complex and controversial at first. Neither soldiers nor hunters favored weapons that might blow up in their face.
Over time, as manufacturing methods improved, cartridges became technologically possible. With cartridges, the gunpowder and bullet are in one piece, manufactured together. This makes them far less likely to blow up in the shooters face. The American Manufacturing Method, using standardized parts, enabled tighter standards.
Today, virtually all guns are breach loading. Some enormous weapons, including mortars, are loaded from the front but these are a small corner case where cartridges would be too big.