I use Rustoleum Universal flat black for the primer/base coat. It sticks to plastics that other do not without adhesion promoter. The spray can top is distictive shape from all the other styles and brands. This paint lays on thick and resist running which is important if you are as ham-handed and impatient as I am when painting.
Step 1: Take toy gun and use about 300 grit sandpaper to remove any logos, copyrights, etc., and also sand any seams you don't want.
Step 2: Then wash with soap and water as MFP 2020 suggested.
Step 3: When completely dry, and the spray can has been shaken thourghly, apply a light coat (just enough to add some color). Wait 15 minutes, apply more paint, 15 more minutes, more paint. Make sure to change angles (the relative position of spray can to gun) and rotate the gun to get complete coverage.
Step 4: Wait 24 hours, flip the gun over and repete Step 3. Wait 24 hours.
You should now have a flat black toy gun. At this point you can do washes and weathering until you get the look you want.
Good luck and let us see how it turns out. :thumbsup