I managed to get a couple of new guns painted last week, both plastic toy guns.
I followed the usual procedure, dismantle if necessary, masking tape used if necessary, everything sanded, embellishments added where wanted. Then a spray coat of acrylic primer followed by a spray coat of matt black acrylic paint.
Then they were both hand painted with acrylic colours and finished with a coat of clear lacquer for protection.
I almost forgot, both of these guns were modified by cutting off the clunky plastic handle which is used for cocking them and replacing them with metal rings which I think look much better.