The O-ring sizes for QB79 / QB78 rifles are:1. Valve -1132. Barrel -012 [qty 3]3. .177 bolt -0034. .22 bolt -0065. Tank block -016 or metric M2 x 15 (15mm ID, 19mm OD, 2mm thick) [qty 2 or 4]6. Transfer port - not an O-ring but a seal. Use poly tubing commonly sold as icemaker tubing (0.25" OD, 0.170" ID) and cut it to 0.100" long. The length is important. You want it long enough to crush slightly to help it seal but not so long that the wall folds and compromises the seal or chokes off the inside diameter. An alternative is Crosman #130-036 which is a square cross section seal. You will need to replace the poly tubing any time you disassemble/reassemble a rifle but the Crosman part is reusable. But then again, you can make about 500 poly seals for the cost of one Crosman seal unless you were going to be ordering from Crosman anyway and can dismiss the shipping charge.For the material, Buna-N is good for HPA but polyurethane is preferred for CO2. A lot of folks get their O-rings from http://www.theoringstore.com since you can buy a la carte and shipping is reasonable. That's probably your best bet if you only need O-rings for one or two rifles. It's generally fine with CO2 as well but some Buna-N O-rings will times will swell O-rings will swell so if you