What Would You Call This?

I am looking for a part that is probably quite common, but I don't know what it is called.  It is 1/2" outside diameter, and about .4" inside diameter.  It has a shoulder on one end so that it doesn't slip inside the 1/2" hole into which it fits.  In this case, it is to enable a M10 bolt to fit snugly into a 1/2" diameter hole.  Of course, this is simple enough that I could make it by starting with a 5/8" piece of aluminum, turning most of it down to 1/2", then drilling a hole through the center.  But I am sure that this is a common item -- if I know what it is called!