Designed and created these darts practice rings similar to those on sale from darts manufacturers. The inner diameter of the rings are between 15 and 50mm in 5mm increments.
I designed these to be used along with a sewing needle or a thumb tack of diameter ~1mm to improve the accuracy of my throw. Simply pass needle through the small hole and attach in a position around the area you wish to hit (eg. treble 20)
If any other sizes are required leave a comment and I'll try my best to update the file ASAP