I have a question about the nomenclature of US small arms.
I know the M1 was the Garand (and of course the M1 Carbine and the Thompson M1 submachinegun) and I know what the M16 is.  I also know about many of the others such as the M4 Carbine, the M14 Rifle and the M9 and M11 pistols.

But what about everything else?

Are rifles, carbines, pistols, etc. included in the overall scheme or are they numbered separately?

Does anybody know what various weapons all the numbers represent, or have a link to a source?

M15 was a stillborn heavy barrell, SAW variant of the M14. Intended primarily for automatic fire, as opposed to the mainly semi auto role of the M14. Cancelled in '59 before any were ever made marked as M15s.

Pistols, carbines, rifles all use their own line of numbers. The current M4 carbine follows by several decades the M1-M3 carbines of WWII and Korea, all variants of the famous M1 carbine. Over on one of the M14 forums Different posted the various rifles that composed the line from M1 to today's M16. Most are obscure, not what you mentally picture when you think of a military rifle. From target .22s to spotter rifles for recoilless rifles, etc.

Quoted: Does anybody know what various weapons all the numbers represent, or have a link to a source?

tri.army.mil/LC/CS/csi/sahist.htm
tri.army.mil/LC/cs/csi/satoc.htm

Quoted: Pistols, carbines, rifles all use their own line of numbers.

I wondered about that, because of the M1 rifle/carbine, etc. but since that was over 60 years ago I didn't know if it had changed - and I don't think it would have become an issue again until the recent M4 carbine.

Most are obscure, not what you mentally picture when you think of a military rifle. From target .22s to spotter rifles for recoilless rifles, etc.

Figured as much, going back to the 1903A3 "aiming device" which was not a small arm, but a "sub-caliber aiming device" for artillery or some such.

There is no rhyme or reason for the military's numbering system.

Quoted: There is no rhyme or reason for the military's numbering system.

That's obvious, after all, we are talking about the government.

I was just curious to find out what all the missing numbers referred to - prototypes, experimental, special-purpose weapons, or applications that we wouldn't even think of as small arms.

Here's what I've found so far:

M1 Garand
M2 Springfield .22 cal training version of the Garand
M3 Infrared sniper rifle version of M1 Carbine
M12     H&R .22LR target rifle (trainer?)
M14 "Improved Garand" service rifle and variants
M15 M14 SAW variant, never produced or issued.
M16 The Black Rifle and variants
M21     M14 Sniper variant
M22     Stoner 63, XM22 never adopted
M24 Sniper weapon based on the Remington 700, chambered in 7.62 and/or 300 Win. Mag
M25     M14 Sniper variant
M40 M40/A1 sniper rifle is based on the Remington model 700 (USMC)
M82 Barrett .50

Anybody know of any others prior to the M16?

Quoted: Quoted: There is no rhyme or reason for the military's numbering system. That's obvious, after all, we are talking about the government. I was just curious to find out what all the missing numbers referred to - prototypes, experimental, special-purpose weapons, or applications that we wouldn't even think of as small arms. Here's what I've found so far: M1 Garand M2 Springfield .22 cal training version of the Garand M3 Infrared sniper rifle version of M1 Carbine M14 "Improved Garand" service rifle M15 M14 SAW variant, never produced or issued. M16 The Black Rifle M24 Sniper weapon based on the Remington 700, chambered in 300 Win. Mag M40 M40A1 sniper rifle is based on the Remington model 700 M82 Barrett .50 Anybody know of any others prior to the M16?

The M3 IR carbine is not included in the rifle number series.  The carbines have their own series:
M1 Carbine
M1A1 Carbine
M2 Carbine
M2A1 Carbine
M3 Carbine
M4 Carbine
M4A1 Carbine

The M24 is in 7.62mmNATO, not .300WM.  It is built on a long action so it can take the .300mag if the Army ever decides to do that, but they're 7.62mm.

H&R built a model 12 for the Army (.22LR target) and it was always referred to as the "M12"

You can add M21 and M25 to the list, both are sniper versions of the M14, but are considered different enough in purpose for their own numbers.

Holes in the system are from weapons that were tested and given an "X" model number, but not adopted.  If they get adopted, the "X" drops off.  For example the XM248 was a competitor for the SAW.  The XM249 won, so "XM248" disappers and we have the M249.  There will just be a hole where XM248 was.  This lets the Army go back and ressurect a weapons system later if it needs to do so without the cost or confusion of renumbering.

The XM-22 was the Stoner 63 rifle.  Since it wasn't adopted, the number was dropped leaving the hole there.

The USMC designated the "M40" so it's not in line with the Army number series, and back then the system wasn't as "streamlined" as it is today.

Prior to the M16, everything was assigned a "T" number for Technical example.  So the M14 was the "T44E4" and the M15 the "T44E5".  No one realy knows why the current numbers start at "M14".

As a side note the M14 was later modified into a support weapon and named the M14A1.  So they spent the money twice for a support weapon based on the M14.