>>The US military is famous for switching job descriptions once people have entered their ranks.
Yes and no.
A contract is a contract. If you sign a Contract for a specific MOS/AFSC (or whatever they call 'em these days) YOU WILL get that OR have the option of declining to remain in the service.
Yes, they might make that hard for you to understand and pressure you to go along with the flow anyways but KNOW YOUR RIGHTS (you still have some!).
In my case I contracted for a specialty position. Even after basic training, even after tech school, if they had decided to pull the rug on that I could have (and would have) walked.
Sure to the uniform dude in front of you this may be incomprehensible, morally suspect, and mean you hate America but to the (big G)Government it is just another set of forms to fill out.
On the other hand if you just sign up without specifics you may as well think of the recruiter as a Marketer. As long as it's not IN THE CONTRACT they are *provably* lying (as they cannot guarantee what is not in contract).
This is where the military gets their reputation having the ability to ignore recruitment promises - most people don't get them written in to the contract.
So my advice is is you WANT to join and have a SPECIFIC goal, get it in writing otherwise the military will put you where IT thinks it will need you.