I am willing to bet that gunpowder was stored there, but not individual arms. Back then, there was no such thing as smokeless powder and it was dangerous to store large quantities in a home. Often times, militias would also have firearms for those that were unable to afford their own weapons. Those are likely to be they type of arms stored centrally, and artillery if they had any.
My home town still has the Colonial powder house sitting atop (strangely enough) Powder House Hill.