Pictured below is a detail I have taken from a photograph of a Volunteer rifle team of the 1870's. All are armed with Snider rifles, and I note that several of them had their muzzle stopper/sight protector fitted, as shown. The full team picture can be found in the "Volunteer Infantry ( 1859-1908 )" forum.
 
David
 

It's not so clear on the first image I posted, but the part pic below (fits to the left of the one above) shows the Snider action more clearly.
 
David