What Helm worn by Reiver ??
Picture if have.

If I understand the question correctly you want to know about these folks: http://en.wikipedia.org/wiki/Border_Reivers ?

If so, perhaps the precise period you have in mind might help. I doubt there is a specific "tribal gear" for these people so for reivers in the 15th century for example you'd have to look at helmets from the 15th century common in that particular area.
But just to be sure, the pictures in that wiki article show burgonetes and the text mentiones morions aswell, though both of these helmets are only applicable from the 1500s onwards.

the same kind of helmets worn by contmporary western european,

there is not a style peculiar to the border of Scotland or england,

sallet for the XVth century.

burgonets, morions and cabacetes for the XVIth century

and the ubiquitous skull cap worn under the bonnet,


i would go for a burgonet or morions, in different level of finish and decorations, depends very much from the status or the wearer

THANK YOU>