I have tried off and on for over a year to cross-breed a barrel cactus. From what I can tell, you need to cross Tri-barrel with prickly pear and you have a 50/50 chance of getting a barrel cactus. I've grown one plant at a time and crossed it with the father plant of the other type. Result: I have 46 Tri-barrel seeds and 48 prickly pear plant seeds. I'm gonna keep trying but this is getting ridiculous! I know I have a "plant murderer-thumb" in RL but I figured I'd have better luck in Britannia!