When saying acronyms out loud, almost always the last syllable is accented (no matter how long the acronym is): US*A*, U*N*, RSV*P*, etc.
Accenting any syllable but the last makes you sound silly (try it). Why is this the case?
One way of interpreting this is that in their underlying form, acronyms have stress on all of the syllables, but the rightmost one is what "wins out" when they are pronounced. According to some theories of English stress, it is a general principle that the rightmost "major stress" in a word receives the primary stress (or the accent). In these theories, words like "facilitate" are analyzed as having either no stress, or only a "minor" stress on the last syllable.
There is some evidence for the presence of underlying lexical stress on non-final syllables of acronyms. When a two-syllable acronym comes immediately before a word that starts with a stressed syllable, the acronym may be accented on its first syllable instead of on its second. E.g. even if "UN" is accented on the second syllable in isolation, it might be accented on the first syllable in a phrase like "UN peacekeeping troops".
Numbers ending in "-teen", such as nineteen, are a well known example of words that may show stress shift and that can be analyzed as having adjacent lexically stressed syllables.
I don't agree with your observation.
I have heard not too infrequently,
Could you please kindly R S ^V P my invitation to your own party?
A mother's anxious request to her daughter for her daughter's engagement party.
The ^U S of A today has won another gold medal.
An excited news anchor.
The ^U N has once again failed to take resolute action towards Syria.
A person resigned to disappointment with the situation in Syria.
^U S - normally the first letter is accented.
^I B M, because saying I B ^M would be awkward.
^U K is more frequently expressed than U ^K. U ^K sounds awkward.
^DD T. D D ^T sounds awkward.
^F B I
Therefore, your observation is not accurate.
Your examples are all properly and idiomatically called abbreviations or initialisms, not acronyms, despite what the dictionary says: There's disagreement about the terminology, as is often the case, no matter what the topic. acronym: a word (as NATO, radar, or laser) formed from the initial letter or letters of each of the successive parts or major parts of a compound term; also : an abbreviation (as FBI) formed from initial letters : initialism. An article on this topic appears in Wikipedia: see the bottom of the article for the world's longest acronym:
ADCOMSUBORDCOMPHIBSPAC, a United States Navy term that stands for "Administrative Command, Amphibious Forces, Pacific Fleet Subordinate Command." Another term COMNAVSEACOMBATSYSENGSTA, which stands for "Commander, Naval Sea Systems Combat Engineering Station" is longer but the word "Combat" is not shortened."
and the world's longest initialism:
The world's longest initialism, according to the Guinness Book of World Records is NIIOMTPLABOPARMBETZHELBETRABSBOMONIMONKONOTDTEKHSTROMONT (Нииомтплабопармбетжелбетрабсбомонимонконотдтехстромонт). The 56-letter initialism (54 in Cyrillic) is from the Concise Dictionary of Soviet Terminology and means "The laboratory for shuttering, reinforcement, concrete and ferroconcrete operations for composite-monolithic and monolithic constructions of the Department of the Technology of Building-assembly operations of the Scientific Research Institute of the Organization for building mechanization and technical aid of the Academy of Building and Architecture of the Union of Soviet Socialist Republics."
I can't find any explanation on the Net, but it seems to me, as Kris commented, that stressing the final letter is an "end marker": The abbreviation's over now, folks.
However, I can imagine someone in Waycross, Georgia, stressing the first letter in UN, just as they stress the first syllable of the word insurance. Can anyone from that neck of the woods chime in on that possibility?