The centennial year of a weapon that may have killed more people than any other is passing unobserved. The weapon is the machine gun.

The main supplier of machine guns to America`s armed forces, Saco Defense Inc. (a subsidiary of the Swiss-owned Alusuisse of America Inc.), is situated in a tidily landscaped suburb of Portland, Me. The quiet of the community is disturbed only slightly by the muffled thumping of machine gun and cannon fire inside the plant`s sound-proof test ranges.

Saco`s three main products are the M-60 7.62-millimeter machine gun (the standard light machine gun of America`s armed forces since 1960); the M-2 .50- caliber heavy machine gun (first made in 1933), and the M-19 40-millimeter machine cannon (first tested at the close of the Vietnam War).

Each of these guns works on a different principle: gas, recoil and blow-back operation. But the three principles have one thing in common: They were all patented by Hiram Maxim between the years 1883 and 1885.

Maxim, who lived from 1840 to 1916, is rarely mentioned in the same breath with Thomas A. Edison, Alexander Graham Bell, the Wright brothers, and the other great American inventors of his day. But in ''The Social History of the Machine Gun,'' the historian John Ellis points out, ''Without Hiram Maxim, much of subsequent world history might have been different.''

The Maine-born inventor never reached the pinnacle of success in his native land, but a turning point came for him during a business trip he made to Europe. He later wrote to The Times of London:

''In 1882 I was in Vienna, where I met an American whom I had known in the States. He said: `Hang your chemistry and electricity! If you want to make a pile of money, invent something that will enable these Europeans to cut each others` throats with greater facility.` ''

Maxim took the advice seriously, set up a workshop in London, and within three years, was building the world`s first practical machine guns. (Most of the earlier rapid-fire guns, notably the multibarreled gun invented by the American Richard Jordan Gatling in 1861, had to be cranked by hand. Maxim`s gun required only steady pressure on the trigger to maintain a continuous firing cycle, including reloading, cocking and the ejection of spent cases.)

During his childhood in Maine, Maxim had been knocked over by the recoil of a powerful rifle, and had speculated on the possibility of putting recoil forces to work to operate guns automatically. In London, Maxim devised a spring-loaded bolt action that could store up the recoil energy released by a shot and use that energy for readying the weapon for the next shot.

Maxim`s ''little daisy of a gun,'' as he called it, immediately attracted the interest of the Duke of Cambridge and British royalty. In one of his machine-gun demonstrations, Maxim impressed the royal family by blasting the letters VR (for Victoria Regina) into a target. The inventor settled in England, became a subject of the Queen, and was finally knighted for his achievements. He was asked to stand for Parliament but declined because of deafness, a problem possibly caused by the 200,000-odd shots he had fired while demonstrating his gun.

The advent of the machine gun meant that for the first time a handful of gunners could subdue masses of enemy infantry, revolutionaries, strikers or hostile crowds. Initially used to help build colonial empires, the Maxim gun did its first large-scale slaughtering in 1893, a scant eight years after the gun`s invention. Fifty British security guards of the Rhodesian Charter Co. in Africa brought four Maxims to bear against Zulu tribesmen, and in less than 90 minutes the guns had killed 3,000 of an attacking Zulu force of 5,000.

On Sept. 2, 1898, the Maxim fought its first real battle when Sir Herbert Kitchener (assisted by a young Winston Churchill) met an army of Moslem fundamentalists at Omdurman on the Nile. The British killed or wounded more than 20,000 of the enemy while sustaining only light casualties of their own. World War I, sometimes called ''the machine-gun war,'' saw carnage unequaled in all previous history. In just one day along the Somme, July 1, 1916, the British suffered 21,000 killed, the great majority by Spandau machine guns, the German version of the Maxim.

Boastful though he sometimes was, Maxim might not have minded the absence of any centennial observances this year for his gun. Toward the end of his life he seemed somewhat chastened by its ghastly effects. ''Had it been anything else but a killing machine,'' he wrote, ''very little would have been said about it.''