Squad level and crew served weapons require special training and earned qualification and proficiency. Mission may drive some options at the squad, platoon, company level. Marine Corps has a Weapons company that has Marines specifically trained on SAW, mortars, and heavier support weapons.
Of course there is cross training to ensure others have good working knowledge to get a weapon up and keep it running if your primary operators are no longer capable. At that point the "decision" phase is removed though.
Unfortunately, there is money to be made by awarding contracts...
The "official" method goes somewhat like this...
Word is put out that a branch or all branches are looking for a new bang-bang.
Requirements are listed. Caliber, reliability, longevity, ease of maintenance, price range, manufacturing capabilities (meaning they want a ready supply), among other requirements.
Different manufacturers go through the process of entering a product for testing and evaluation. Changes are often made and deals tweaked by the evaluation teams (DOD, Ordinance board, who the hell ever they are)
And then, the one that meets all requirements the best is the one that's supposed to get the contract. (read my first statement).
-A need is indentified
-Then a requirement is issued by the respective service (Or a "warfighter challenge" which is not quite a requirement, but a solication for examining the need and ID'ing possible solutions)
-Then its examined if a solution exists
- and then it examined if its critical enough to fund.
- If yes then a market survey is conducted and manufactueres are invited to submit competing proposals
- Proposals are compared, often by competitive evaluation (thugho sometimes not, if time is critical)
- Then the winner gets an order
(if no existing solution can fill the gap AND it is ID'ed as critical AND funds have been made available then a development contract may be issued).
When you enlist, your can choose your job (to an extent). You can sign on to be a mortarman, rifleman, artillery, whatever. Then you'll get issued a weapon depending on the units needs within your specialty.
I was a SAW gunner my entire enlistment. I enlisted as a rifleman. My unit needed more SAW gunners when I joined up, and so I was told, "hey you! Yeah, you, the boot! You're a SAW gunner now."
depends on the unit. sometimes its hey your being a retard you carry this...
the better units assign weapons based on aptitude with a given platform. it turned out i was an artist with an m60 so my platoon sergeant made me a machine gunner. i was also really good with a m203 m16 saw mk19 but i preferred the 60.
the one thing i thought was stupid was that you had to be a sergeant to get a 203 regardless of any ability with the firearm. our unit only one e5+ had any ability to use a 203 at all. we had many e1-e4 that were reallllly good with the launcher.