lube it up good with silicon oil. then ive found that a small ball-end allen driver works good to manipulate it in .. push one spot into the groove so it catches then go to the side sticking up and push that spot down and snag it on the groove .. the rest can then be pushed down and it should snap in pretty quick ... you need the silicon lube.. and you need a blunt thin instrument to push it down, nothing sharp ..
If you have a non-metal cleaning or other rod that can go in from the muzzle end and stop just short of the O-ring grove. Gives you something to push against. A cleaning pellet can work, too. The narrow mag slot makes it more difficult to get it in. If need be, the barrel is not hard to remove.Crosman calls out OD 8.2mm x W 1.5mm, which in the more commonly listed as CS 1.5mm X ID 5.2mm. It is not a common size though. PS: I just checked the spare in my bag and it looks like a 1.5mm CS X 8mm ID. Way too big for the barrel inner O-ring. I think it might be for the included female foster fitting.PSS: You might try a -009 O-ring. It is what the 22 Mrod uses. Many hardware stores will have it. 7/32"ID X 11/32"OD X 1/16"CSActual: 0.208"ID X 0.070"CSMetric: 5.28mm ID X 1.78mm CSIt is a little large in the OD, but might work.