Hi,
As far as i know, all MP40 were gloss/satin black finish.
To add a used look, you can apply the drybrushing technic on it. It works fine with plastics model kits.
Dip lightly a brush in silver paint, wipe it with a rag, or some paper towel, then scrub the brush on edges of your MP40 model. When it's as you like, use a spray varnish (i prefer satin finish) to protect your paint.
My apologizes if it's not clear enough, these days my english is a bit lame.
Regards.