The lead cap from a powder-holder of the 17th century. The cap was originally of circular cross-section, but is now slightly crushed. The base is flat and has two integral loops at its edge, both now broken.
Powder holders were made of wood, and were sometimes called 'apostles', because there were normally twelve of them suspended from a soldier's bandolier. They each held the correct amount of gunpowder for a single charge. Both the holders and the caps had loops (normally two) through which the attachment cord was threaded, and which prevented either from being lost. When the powder had been used, the holders were refilled from a flask.
Lead caps of this type are often found on Civil War sites.