The picture helped, thanks. I remember replacing a set of 8 of those orings on different mags, they're a bitch to get on. I ended up actually making a special "tool" to get them on lol, i'll take a photo for you in the morning.
One of these is your oring:
4 - 81-9119 CO2 Housing O-Ring - 105-P90
5 - 45-9109 CO2 Housing O-Ring – 006-P70
Fairly sure its the 006-P70, but they cost like 50c so why not get both and be extra sure?
In the mean time, try using a pen. With the tip of the pen out, roll the oring onto it wider part of the pen, and then retract the tip back inside the pen. Put the pen up to the stem with the oring still streched on it and try to roll it onto the stem, straight from the pen so it stays streched all the way.
If that makes no sense just gimmie 24 hours and I'll get back to you with photos of how i did it and a picture of the ghetto tool i made to do it with.
Also, if you wanna shell out the 5$ rockstar sells the piece with the oring already on haha: Tiberius Arms (T8/T9) CO2 Valve - 45-3203