Just FYI, I am getting many of these springs custom made for my up and coming 6 position recoil tube, in T302 stainless spring wire, the idea that it sells for $22 on some retailer websites is almost criminal, I know how much these are to make...
It isn't quite worth the hassle for me to sell them individually to those that want it, but I might sell packs of 4 or 5 for $10-$15 in the future for spare parts if there's demand for it. Fastenal and Mcmaster does not have the size that is needed. The ring is actually around 17.8mm OD free, and 15.5mm ID free. The gap is about 8.5mm or so. when compressed, the ID reaches 15mm inside the tube, to retain the plunger.
-Robb