This one is actually very, very easy to find a POD for.
The United States, as we know, fielded the M1 Garand as its infantry rifle in WWII. Like most forces, they found the gun unwieldy for paratrooper operations and the like. Unlike most, they also realized that the infantry rifle round was grossly overpowered and looked for a lighter and handier weapon. They eventually put together a weapon which was (as originally envisioned) selective fire and shooting an intermediate round. They removed the ability to shoot automatic from the original design, but reintroduced it in 1944. Troops were very split on whether they liked the Garand or the new gun more.
What did they do wrong on the M1A/M2 carbine? The mistake was small but telling: when the Germans and Soviets made their assault rifles, they cut down a rifle bullet. The American gun juiced up a pistol round. That was the error.
So, someone designs a shortened .30-06 much like the Germans made a shorter version of their rifle cartridge and they use that in the carbine. Instant US assault rifle... in 1942. And in large quantities.
Actually, the OTL M1 grew out of an interwar program examining a .276, which Garand could never get to feed properly (because of the lubed ammo?), while CoS MacArthur demanded it use the .30-'06 round 'cause the Army had such large stocks of ammo...

Suppose it'd occurred to Garand to shorten the .30-'06 & neck it, making a .276-'06 Short (7x40mm).


It could enter service as early as 1931-2 (instead of '36

OTL) & be more/less standard across the U.S. military (including the MC?) by '40. It'd still have problems with the lo ammo cap (maybe 10rd,
versus OTL 8) 'cause the Army demanded a mag flush with the furniture,

& the auto-eject when empty,

both of which could easily be cured with a 20-30rd M14-style mag as hi-ROF engagements showed 10 isn't near enough... And to save money, simple metal folding stocks, which would also be useful for paras, tankers (due to close quarters, retain the 10rd mag?), & so on... Plus, with hi-ROF, accuracy is less a concern than laying down fire, so even troops like clerks & truck drivers (10rd mag, still?), for whom the M1 Carbine was mainly intended (pistol not powerful enough, M1903 too unwieldy), could use it successfully.