The second Amendment reads;
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.

it is clearly within the framers' mind that the right to bear arms derives from the notion that this right descends from the effectiveness of a militia- which paralels similar English legislation of the time. it should be noted that, technically speaking, the (English) British did not have a standing army until the 1870s- no such entity legally existed. where British and American practice diverged was in the 1800s pursuit of western exploration and colonisation by the US in the wake of the Louisiana Purchase; the Mexican and American War; and the Oregon Settlement. Militias were not feasible on the new frontiers but the personal right to self-defence against various agencies (Amerindians, rustlers, bandits) meant that maintenance of a right to bear arms no longer derived from keeping an effective militia. this required a change of thinking from that which had maintained in the 1770s when the constitution was originally written, largely inspired by English Enlightenment notions of political theory.
I am not arguing for a day to day re-interpretation of the constitution and the Bill of Rights. while the left is pushing a political agenda that claims, in part, to be the sole representative of progress, the right is stuck in the mud trying to present a united front and backing away from supporting any change. the moderate right needs to redefine itself as a supporter of sensible progress even if that means alienating its more radical members. then it can more effectively challenge these unnececessarily liberal interpretations of what progress actually means. using the constitution as a buttress against change, when it is clearly designed to be open to changes of interpretation and necessity, is simply playing into the hands of the liberals.
whilst the M303 is illegal, this is by no means the sole interpretation of a grenade launcher. a look at some of the ammunition types being developed in Europe and North America for even 5.56mm calibre weapons include several 'exploding' bullets. one bullet being developed for the .50in Barrett rifle, itself deadly in the hands of an expert to over 2 kilometres or one and a quarter miles, by a niche company include flechettes that deploy inside the body. to have to name every bullet or weapon that you want banned, as opposed to effects produced, means a greater focus is placed on a universal ban- which is precisely what happened in Australia.