The US Army replaced the M1911A1 with the M9.
The M9 was the first pistol adopted since the Army changed the designation system from Myear to Msequential.
The change from year to sequential was in the 1920's or '30's (I can't remember exactly).
The new pistol that replaces the M9 is the M17.
If you had small hands, there was a SIG P228 designated M11.
What were pistols M1 through M8, M10 and M12 through M16?
This is not the only time there's been massive gaps in the system.
For machine guns we drop off with an M3 (Browning .50 cal variant) and don't pick up again until M60! Then we hop to M73 then M85 then M219 then M240 then M249.
Carbines is mostly complete.
M1, M2 and M3 are all variations of the M1 carbine in .30 Carbine. M4 is the stubified M16. M5 through M7 disappear before the XM8 surfaces.
M1 then M14-M16 then M21, then M25 then...
It can give one agita.
It seems like there should be a list someplace where these guns have been type classified, but not adopted. But I routinely fail to find it. XM8 is probably a clue, classified, tested, not adopted.
I wonder if the late '80's ACR program contenders all have XM numbers assigned, but not widely publicized.