Time for a tutorial. From the first days of U.S. military arms, the Army called weapons by their date of adoption. So we had the Model 1863 (rifle musket), Model 1873 (rifle and carbine), Model 1903 (rifle), Model 1911 (pistol), Model 1917 (rifle, machinegun and revolvers) and Model 1928 (submachinegun).
In the early 1930's, the U.S. Army decided to start all over and name each major item in a new series, each starting with M1. (The Army claims the "M" doesn't mean "Model", it is just a designator.)
From that point on, as a new item was adopted, it became the "(whatever) M1"; the second type became the "(whatever) M2", and so on.
Some confusion is caused by ignorance of the major type. For example, the Light Tank, M3, was the Lee; the Medium Tank, M3, was the Stuart. (The famous Sherman was the Medium Tank, M4.) The first M1 tank is better left unmentioned; even by the standard of the time, it was a bad joke.
So the first rifle adopted under the new system was the Garand, which became the Rifle, M1. The Winchester light rifle became the Carbine, M1. The old Thompson SMG was the Model 1928A1, but when modified, it became the SMG, M1. Everything got the same treatment - weapons, helmets, slings, bayonets, web belts, boots, mess gear, and so on.
But in later years, some numbers became cumbersome, so the Army started all over again, with items like the (new) M1 tank, a far cry from the original with its 37mm gun.
But the rifle adopted as the U.S. Rifle, Cal. 30, M1, became so ubiquitous that it became to the troops just "the M1". No soldier called it the "Garand" and no one bothered to say, "rifle"; the M1 was "the M1". If you meant an M1 carbine, you said "carbine". If you meant an M1 helmet, you said "helmet".
But the U.S. Army has no copyright on the term "M1". So I can produce a .22 pump rifle and call it the JK M1. Or I can make a hair dryer and call it an M1. Some combinations of manufacturer and model have been determined by BATF to identify machineguns, so to avoid legal problems makers of similar non-machineguns change part of the name. The best known example is the U.S. Rifle, Cal. 7.62mm, M14; as made by or for the Army it is a selective fire weapon and a machinegun. But a company may make a look-alike semi-auto rifle as long as it is marked with the name of the company which is not a government maker or as long the model number is changed.
Hope this helps or maybe just makes things more confusing.
Jim