Solid brass swirl-head "ice cream cone" rivet for use on 15th century armor. These were found in a few places; holding in the liner in sallets and some barbutes, on besagews and in the center of elbow-cop wings, or, use your imagination! 

The head is 1/2" across, with a 1/8" x 1/2" rivet in solid brass. 

These need to be carefully piened into lead or similar material or you can deform them.