You got really lucky! In MA possession of ammunition without a (MA issued) permit is a crime with up to 2 years in jail.

MA doesn't have ANY reciprocity with other states regarding your CCW and the City of Boston in particular has a ban on any high-capacity (>10 rounds) magazines of ANY sort/age. MA banned hi-capacity mags but only ones made after 1994, pre-ban ones are ok, just not in Boston (and I think a few other towns).

You are right about peaceable journey, but I would not risk any of my liberty or rights on the good nature of a LEO in such a state.

I believe you are mistaken, as so very many people are, about that document. If you are quoting an important legal document, it's important, wouldn't you agree, to get the exact text correct, otherwise the meaning might change? Especially if the text is used for justification to allow a wide, practically unregulated population to be armed? Let's see, I am willing to bet the following statement is the actual quote from the document you reference, which says something quite different (to me). Here it is: "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed." nevermind that the more legal gun holders there are, the more opportunities for criminals to get guns. Legal gun owners routinely do not secure their weapons in the manner prescribed for responsible gun ownership(I grew up in the south, I can count the number of gun safes I've seen in houses on 1 hand). In fact, many decry the safeguards as imperiling their lives should a crisis spontaneously occur. I argue that those individuals are in fact responsible for a non-negligible amount of crime, since their actions (purchasing a firearm and then failing to secure it against threats) directly leads to more gun crime. But undoubtedly these reasonable arguments and facts will do nothing to move your position, so I guess I just wasted both our times.

TSA agents are federal employees and federal law prohibits carrying-on ammunition on planes. It is not Boston specific other than that's where OP got caught. You can still check it if you want. I assume the reason is to reduce the chances of accidental discharge causing risk to the plane and/or the prevent one part of a weapon that could be used for hijacking. Even if laws allowed you to take bullets into airports, I have no doubt that the airlines (private business people choose to patronize) would prohibit you from carrying ammunition onto their planes and they'd have every right to do so.