I use a wood dowel from the muzzle end, I have a clamp on the dowel , so it stops when it is flush with the o-ring groove edge.
fold the o-ring and shove it in the breech end and then use the dowel to push back on it, as you use something short (like an allen wrench) on the breech end to force/nudge the little burger in to the groove, work it back and forth a little, pre-lube with silly-con oil,,, forgive my humor