Also just a word of warning:
Lasers from eBay and other sketchy websites are almost always >5mW, so if you plan of actually pointing this thing at people, you should get something that is certified to be <5mW. This will cost quite a bit more, but you can't put a price on eyesight. There's also the fact that cheap 532nm modules leak IR, which is invisible, but can still damage your eyes.
rocknrude --- The above is extremely important---you need to be 100% certain about 5mW or less and emits no IR meaning any IR is filtered out if this toy is going to be pointed at people who are not wearing laser goggles/glasses and there is a possibility of hitting people in the eyes with your "laser sight", the only way to do that is to test each and every unit on a Laser Power Meter.
Keep in mind also the even 5 mW or less can cause eye damage and for that matter so can 1mW.
"The hazard distances listed below are intended only as general guidance. This is because 1) your laser may vary from the parameters (power, divergence) listed below, and 2) information on labels or marketing materials may not always be correct. For example, studies have shown that some laser pointers may be falsely labeled to avoid regulations -- the actual power may be 10 times or more what the label indicates.
Always err on the side of safety. If your laser has not been measured by a knowledgeable and trained Laser Safety Officer, assume it is more hazardous than the label or marketing materials would indicate."
Class 3 is a certified less than 5mW of visible wavelength only laser---no IR and here is the risk:
EYE INJURY HAZARD -- DIRECT AND REFLECTED BEAM
Class 3R visible-light lasers are considered safe for unintentional eye exposure, because a person will normally turn away or blink to avoid the bright light. Do NOT deliberately look into or stare into the beam -- this can cause injury to the retina in the back of the eye.
Be aware of beam reflections off glass and shiny surfaces. Depending on the surface, the reflected beam could be about as strong and as focused as a direct beam.
The Nominal Ocular Hazard Distance (NOHD) for the most powerful Class 3R visible-beam laser (4.99 mW) with a tight beam (0.5 milliradian divergence) is 104 ft (32 m).
Even Class 2 less than 1mW laser of visible wavelength only , no IR output ---can damage eyes
"EYE INJURY HAZARD -- DIRECT AND REFLECTED BEAM
Class 2 visible-light lasers are considered safe for unintentional eye exposure, because a person will normally turn away or blink to avoid the bright light. Do NOT deliberately stare into the beam -- this can cause injury to the retina in the back of the eye.
Be aware of beam reflections off glass and shiny surfaces. Depending on the surface, the reflected beam could be about as strong and as focused as a direct beam.
The Nominal Ocular Hazard Distance (NOHD) for the most powerful Class 2 laser (0.99 mW) with a tight beam (0.5 milliradian divergence) is 46 ft (14 m)"
See:
Safety of Class 2 visible-beam lasers
and
Safety of Class 3R visible-beam lasers
and
http://www.lasersafetyfacts.com/resources/Spreadsheet---laser-classes.pdf
A good chart of Laser Hazards can be found here:
http://www.lasersafetyfacts.com/resources/FAA---visible-laser-hazard-calcs-for-LSF-v02.png
You would be much better off, meaning creating something with less risk of damaging someone eyes, with a Class 2 1mW or less than with a Class 3, 5mW or less laser .
Either way unless the "toy" you are creating complies with FDA rules and regulations" you are in violation of US Laws, rules, and regulations pertaining to lasers . If you don't know what you are doing you are an "accident looking for a place to happen" if your Nerf toy with a laser sight is going to be pointed at or around other people who are not wearing laser safety glasses or goggles.
You will be personally liable for any accidental damage your laser sighted toy does and additionally subject to punishment for violation of FDA laws, rules, and regulations.
See:
https://www.fda.gov/downloads/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/UCM363731.pdf
and
https://www.fda.gov/radiation-emittingproducts/radiationemittingproductsandprocedures/homebusinessandentertainment/laserproductsandinstruments/default.htm
If your "Nerf toy w. laser sight" involves use by children/minors meaning individuals under 18 years old forget about making it altogether.
If you want to see people. meaning adults with real capabilities and resources to hunt you down like a dog, get extremely angry and aggressive very fast/very quickly make and distribute something that points laser beams of any output at/into children's eyes. Common sense should tell you that. :crackup:
My advice is forget a laser sight on a Nerf gun toy to illuminate a target if you want to avoid the possibility of serious problems and trouble being you know little, if anything, about lasers, the safe use of same, and the FDA laws, rules and regulations pertaining to lasers. Know that what flies in China does not necessarily fly in the USA---so buying from lasers China ignores all USA laws, rules, and regulations, generally speaking. 
