Okay, so a little self-referential blahblahblah on Bucky’s NCO career, mostly as a follow-up to the Sam Wilson Is Not an Officer stuff.
(1) At the start of Captain America: the First Avenger, Bucky has been a soldier for a while and a very good one. Whether Bucky enlisted or was drafted, he went to basic training and he emerged some flavor of private or, in truly exceptional circumstances, a corporal. Nobody comes out of basic a sergeant, which is an NCO (non-commissioned officer) rank and one of responsibility. When we meet Bucky in the movie, he’s been a soldier for a while, long enough for at least one promotion up to E-5, two or three promotions being much more likely. Which is a lot in a short amount of time – about a year-and-a-half past Pearl Harbor, less time in service assuming Bucky didn’t ship off to basic in 1941. As such, I’ve usually written Bucky as getting a field promotion for valor in combat because things just don’t happen that quickly. It’s still a speedy trip to sergeant, but it’s not completely ridiculous.
Any way you want to play it, when Steve is asking Bucky if he’s gotten his orders, he’s not asking brand-new-soldier Bucky about his first chance to be a ‘real’ soldier. He’s asking probably-home-on-leave Sergeant Barnes where he’s going next.
(2) Bucky has experience leading small units – a team, a squad. He might have already been a platoon sergeant, but no sure thing. Regardless, by the time he’s rescued by Steve, he’s an experienced NCO. He knows how to get things done, both with respect to regular Army crap and the corralling and maintenance of the men in his unit. He understands how the division of labor between CO and NCOIC works out, that he is the sheepdog to the CO’s shepherd when it comes to executing orders and handling the men. He also understands that the relationship between platoon sergeant and platoon commander is a separate thing between them and has a public face, which is united and in which the NCO is proper and respectful of rank, and a private face, which is more informal and generally reflects the fact that the NCO has more life and military experience than the officer and has an obligation to use those experiences to improve the officer and keep everyone from getting killed.
(3) Both points above matter when it comes to Sergeant Barnes and Captain Rogers, especially because the latter was commissioned as a captain and has never had a command position before at any level and truly and completely knows nothing about nothing about leading anyone anywhere to do anything in some form of proper military fashion. Bucky’s instruction necessarily doesn’t begin once he’s team sergeant on the Howling Commandos – it begins during the rescue, the minute he realizes that he’s not having a drug-induced hallucination and Steve really is Captain America and needs all the help that he can get because Steve doesn’t know what the hell he’s doing. Even if Steve doesn’t confess that right away, which he probably will, Bucky knows him well enough to tell.
(4) As important as Sergeant Barnes’s experience is to Captain Rogers, it’s possibly even more important to Brooklyn’s Own Bucky Barnes. Who has been through hell on the battlefield, an even worse hell in Zola’s and Schmidt’s lab, and is now presented with a very hard truth: Steve Rogers doesn’t need him anymore. Steve is no longer frail by many metric; he doesn’t need defending or nurturing, he doesn’t need anyone to advertise his virtues or prop up his self-esteem because everyone else now knows exactly how awesome Captain America is. Steve is no longer short of friends or invisible to women or at the mercy of either his ailments or the neighborhood bullies. Every single protective function Bucky has ever filled for Steve out of friendship and brotherhood has now been rendered moot. Thankfully, while Steve may not need him for anything but companionship anymore, Captain Rogers needs him for a hell of a lot. Steve may be quicker in mind and body, but Bucky is the one who knows how to make everything happen. And that won’t change even as Steve learns the ropes; Captain Rogers will always need Sergeant Barnes. And that’s probably a comfort to Bucky at a time when little else is.
And now the self-referential part, because I’m like that:
Antediluvian and La Caduta: the former is Bucky’s pre-movie war career and the latter is his imprisonment, where he struggles to be an NCOIC while also being a lab rat, through the rescue and the formation of the Howling Commandos.
Recursive, which is a Steve(-and-Bucky) story, but mostly about the Howling Commandos and Steve’s CO-NCOIC relationships with both Bucky and Dum Dum Dugan (after Bucky’s fall) matter a lot.
While I really like this version of Bucky’s history, I want to point out that the general premise - that Bucky would not have become a Sergeant without a fair amount of field experience and likely a battlefield promotion, and that he would have been promoted unusually quickly - is not necessarily correct. The US’s entry into WWII resulted in such a massive and rapid expansion of the US military that it was simply not physically possible to promote NCOs based on experience - they needed new NCOs too desperately for that. New recruits and draftees were given an aptitude/general intelligence test, the AGCT, and the top percentiles of the scorers were designated as NCO candidates (actually, at the start of the war, the top scorers were sent to the Army Air Force or to college for officer training, so the above-average-to-middling-ish were left to become the NCOs for the new divisions; being a high school graduate would likely be enough to make you a candidate in some cases.) Very often NCO candidates would be promoted to Corporal right out of Basic, get minimal extra training, and make Sergeant very quickly thereafter; some people selected for extra specialty training would be promoted to Sergeant by the end of the extra training. And in 1942, when the number of enlisted men was increasing by hundreds of thousands a month and the need for NCOs was desperate, it wasn’t unusual for a particularly promising recruit to be promoted directly from Private to Sergeant while still in boot camp.
So it’s possible Bucky has some combat experience and a field promotion, but as long as you believe he could have gotten, oh, somewhere around 100 on a primitive IQ test (and that he had enough pride that he would’ve turned down the chance to go to college instead) it’s also perfectly likely that Sergeant Barnes really was just out of training and about to go overseas for the first time at the beginning of the first movie, and was in over his head almost as much as Steve in terms of actual knowledge and experience - but figured it out as fast as he needed to in order to keep being Steve’s backup.