The Supreme Court disagrees with you. No predicate (or in this case, prefatory clause) could undermine or limit an operative clause that straightforward, at most it could be considered a motivation: i.e. the right of the people to keep and bear arms is intended to (at least, but not solely) give those people the ability to keep and bear arms sufficient to feasibly form an armed militia.

Well if you listened to Heller vs DC, the court decided that it wasn't a predicate based on the interpretation of preceding and subsequent state law and other historical reasons.