I wrote this out to answer this question elsewhere and thought it might be good to store it here.
IF you break/rip/tear and O-Ring and you need to find a replacement for it, here's how you can do so.
1. Use a ruler to measure the length of the broken o-ring piece inside of the curve.
2. Use that length with this calculator set to "Given C" http://www.calculato...lane/circle.php
3. Write that number down as you now have the actual ID of the O-Ring (or close to it).
4. Measure the thickness of the O-Ring as best you can. This number will be close to either a fractional size, or a millimeter size. It's best if you can use a caliper to get this number, but a ruler with both inches and millimeters will do. Always make sure to round DOWN to the nearest fraction or millimeter since o-rings are oversized. For example a "3/16" cord width O-Ring is going to have a NOMINAL width of .210".
5. Write that number down.
6. Take both numbers and poke through the McMaster catalog in the o-ring section. You can find the nearest ACTUAL SIZE for Inner Diameter from their list as well as the ACTUAL (or fractional) cord width.
Just pick BUNA-N as your material for now. The majority of the o-rings used for Nerf are that material.
Now you have the DASH NUMBER of the o-ring you need. You can order the o-ring you need from McMaster, Amazon, Fastenal, or buy it from a hardware store. If you want to buy the o-ring from Home Depot you may need this chart to convert from DASH NUMBER sizes to the Danco brand o-ring part numbers.
Other brands may have different part numbers so check your nearest hardware store's online catalog before going to see what brand of o-rings they sell. If they don't list the products in DASH number sizes just e-mail the manufacturer and ask them for a similar conversion chart.