Subsidiarity can be a bit more complicated than the freedom of a silly person to do what he wants. A conscience per se means nothing: it has to be an informed conscience. Subsidiarity refers to the level that is likely to be the best informed about whether or not to do something. While you may be at the level of subsidiarity that is best informed about whether you should go to the bathroom before stepping onto a municipal bus, the level of subsidiarity that is best informed about whether you should be taking the bus trip at all may be your parent or the civil emergency authority in your town.
Is the 'competence to know what is the wise thing to do' in regards to owning a dozen AK47s at the subsidiarity level of the mind of the single gun collector or does it rest with the many minds in a legislature?
Why is there a necessary logical requirement for you to defer to John Doe's desire to own an arsenal when your own risk assessments suggest that your physical safety could be impaired? Is his brain better than yours? Why isn't there a necessary logical requirement for you to be a rational steward of your own interests and advocate to the legislature that they restrict John Doe's ability to own an arsenal? Rights under law (except for constitutional rights) aren't static. If they were, there wouldn't be lobbyists and there would be no need for an NRA. Why can't you argue that the status quo allowing John Doe to amass an arsenal be changed and let him argue the opposite?
If an individual at my level cannot be competent in regard to what is wise in firearm ownership, then it is not possible for a legislator to be competent in that regard. There is no ontological change in a person who becomes a legislator, his humanity is interchangeable with mine.
This does not mean that there are not crazy people out there who do bad or unwise things with firearms, but until he's proven incompetent and criminal, he cannot be restricted as if he is incompetent and criminal. This has nothing to do with one brain being better than another. It has to do with subsidiarity and prudential judgment. Each man, in order to be good and moral, must know what it takes to do his duty productively and spend his liberty in a recreational or refreshing fashion. Until I have a good reason to believe that John Doe is not a good and moral man, I have no good reason to fear him if he owns an arsenal. In fact, to entertain such an unreasonable fear on an intellectual level would be quite uncharitable. To go even further and lobby that his ability to own a specific type of private property be restricted for no specific good reason (but rather for a "possible" reason, which may never in fact exist) is a usurpation of his natural right over his own private property.
Rerum Novarum applies this notion of right to property to extend in an even stronger fashion over heads of households as a sacred law of nature. To quote the encyclical: "Provided, therefore, the limits which are prescribed by the very purposes for which it exists be not transgressed, the family has at least equal rights with the State in the choice and pursuit of the things needful to its preservation and its just liberty. We say, 'at least equal rights'; for, inasmuch as the domestic household is antecedent, as well in idea as in fact, to the gathering of men into a community, the family must necessarily have rights and duties which are prior to those of the community, and founded more immediately in nature. If the citizens, if the families on entering into association and fellowship, were to experience hindrance in a commonwealth instead of help, and were to find their rights attacked instead of being upheld, society would rightly be an object of detestation rather than of desire.
"The contention, then, that the civil government should at its option intrude into and exercise intimate control over the family and the household is a great and pernicious error. True, if a family finds itself in exceeding distress, utterly deprived of the counsel of friends, and without any prospect of extricating itself, it is right that extreme necessity be met by public aid, since each family is a part of the commonwealth. In like manner, if within the precincts of the household there occur grave disturbance of mutual rights, public authority should intervene to force each party to yield to the other its proper due; for this is not to deprive citizens of their rights, but justly and properly to safeguard and strengthen them.
"But the rulers of the commonwealth must go no further; here, nature bids them stop. Paternal authority can be neither abolished nor absorbed by the State; for it has the same source as human life itself. 'The child belongs to the father,' and is, as it were, the continuation of the father's personality; and speaking strictly, the child takes its place in civil society, not of its own right, but in its quality as member of the family in which it is born. And for the very reason that 'the child belongs to the father' it is, as St. Thomas Aquinas says, 'before it attains the use of free will, under the power and the charge of its parents.' The socialists, therefore, in setting aside the parent and setting up a State supervision, act against natural justice, and destroy the structure of the home. "
To sum up, if a man sees fit in his prudential judgment to own a firearm or many firearms for his own protection, or the protection of his family, that's his private business until he's convicted of actual criminal behavior.