We had a Cap-Chur gun when I was in practice. Hated the dad-blasted thing. Only ever used it for tranq'ing cows that folks couldn't catch - 70% of my cattle clients did not have cattle handling facilities - no corral, no chute, no headgate; at most, some might have an old tobacco barn... if they could fool the cows into going in there.
Had way too many instances where I'm certain that it didn't deliver the full dose - or any of it, for that matter.
You couldn't count on it - one cow might be out like a light in 5 minutes and sleep for a day and a half, the next might slow down enough in 30 minutes that if you were real stealthy, you might be able to drop a rope on her - then she was up and on the fight!
Been hearing some reports of cattle showing up at slaughter with medi-dart needles broken off in them. Y'all be careful.