608 bearings, press-fit, to be used with 6mm airsoft pellets.
Designed this as a printable substitute for this common bearing as I found other printable designs to be flawed for situations involving rapid rotation. With a .4mm nozzle some areas will be only a single wall thick, so this bearing likely won't be reliable in situations involving any significant force, but I've had great success using mine with a ~400g load for a week now. Prints well with PLA, but if you're able to print fine detail in ABS or PETG you'll likely get a more reliable bearing.
Print the bearings, fill the outer ring with seven airsoft pellets, place them along the rim, and place the inner ring on top. Grab a book or other sturdy, flat item, and give it a solid smack. The bearing should snap together.
If you're printing only a single bearing you may wish to run your printer at a lower speed, to avoid warping. The tolerances are quite strict, the bearing exhibits significant resistance with even slight warping.