Good point. The anti-gunners are always claiming that the pro-gunners ignore the preamble over the main clause. Of course, we do not read the preamble as a "restriction" on the right (and thereby transforming from a right into a government granted privilege). On the contrary, we the see the preamble precisely as an essential justification for the right. Consequently, we must also protect our right to train and to aspire to the level of capability of a "well-regulated" (as you say "well-trained") and effective militia.
I dare say this was the originalist understanding of this constitutional protection. And it scares the crap out of the left.
"What, Sir, is the use of a militia? It is to prevent the establishment of a standing army, the bain of liberty. When governments mean to invade the rights and liberties of the people, they always attempt to destroy the militia, in order to raise an army upon its ruins." Elbridge Gerry, August 17, 1789