Complex sentence: The Lebel rifle is the first military weapon ever designed to fire ammunition loaded with smokeless gunpowder: it had just been invented in 1884 by French chemist Paul Vieille (1854--1934).
Split into simpler sentences: The Lebel rifle is the first military weapon ever designed to fire ammunition loaded with smokeless gunpowder. The latter had just been invented in 1884 by Paul Vieille (1854--1934).