bothhavesharpteeth:

That’s incorrect, there wasn’t a restriction on non-commissioned rank promotions during WWII, the need for officers was too great to restrict it in such a manner.

Also, during WWII ‘enlisted ranks’ stood to mean the entire pool of servicepeople, both drafted and voluntarily enlisted, because during the period from 1940-1947 (under the Selective Service System) nearly 10 million people were drafted into military service.

Bucky could have – and most likely was – accelerated through the ranks because of his skill as a marksman.

The serial number Bucky uses during the scene in Zola’s laboratory in The First Avenger – 32557038 – indicates that he was drafted in New York state after 1942.  However, the Smithsonian exhibit in The Winter Soldier says that Bucky enlisted voluntarily following the attack on Pearl Harbor (December 1941), meaning his serial number should’ve begun with a ‘12’ to show that he was recruited voluntarily from New York state before the military serial number range was extended to 30,000,00.

In the great words of every Marvel fan ever: “Lol, what’s a continuity.”

OP conflated a few things that sorta got brushed over in the explanation?

There are two broad categories of ranks in the military – enlisted and officer. Enlisted within that have NCOs which literally stands for Non Commissioned Officer.  Officers have commissions. (Why this exists the way it doesn’t in ancient military history and all about buying a rank from the local nobility hence the term commission and not really relevant here.)

In the US Army in WW2 the ranks started at Private, then Corporal, then Sergeant (in all the flavors like Sergeant, Technical Sergeant, Sergeant First Class, First Sergeant etc). 

There were at the time two main ways people got into the service – they were drafted or they enlisted (yep, there is that word again and that’s what leads to the confusion). You could be drafted and still enlisted. There was a third way which was that you could commute your jail sentence into military service but people don’t like talking about that one.

It was also possible to jump from enlisted to officer without going to OCS (Officer Candidate School) and that was to either get a college degree and re-enlist or you could get a battlefield commission. 

There is a third class of officers that movies skip over unless they have aviation in them which are warrant officers. These days the only warrant officers left are pilots. Essentially this is the military saying you have an expertise in a skill. The difference between Warrant Officers and Commissioned Officers? Warrants don’t have to run a company of men, they just have to be really, really good at their jobs.

Ranks have evolved over time. For example, technical sergeants don’t exist any more and there used to be a lot of different types of warrant officers. Each US military branch also has its own rank structure so you can’t assume a sergeant on one service is actually the same rank as a sergeant in another.  And of course the minute you add in the mix of services that were in the Howling Commandoes (the movie version) it gets complicated pretty quickly.