I think we've strayed from the fact that JackieE was not using the chain bit (which is still severe, IMHO), but the chain of a leadrope. The difference with that, is a chain bit can only be so slack (and can only pinch/injure to a certain point) while a chain leadrope can get quite slack and then suddenly go taut, which can result in severe pinching and some injuries to the mouth.
As I said before, if this colt (after 4.5 months of training!) does not know what "whoa" means, chances are he doesn't know how to lunge/be lunged properly as chances are was jerking on the lunge a lot. I know from experience some of the horses I had the.. uhm, joy of working with and lunging, that it takes a lot of time and patience to get them to actually be supple on the lunge instead of pulling against it to head in one direction, or to come in too close into the personal space of the "lunger". So, until JackieE comes here to clear things up, I'm coing to assume that there was a lot of angry yanking involved with "lunging" that colt as her post does sound like she lost her temper.

I find it amusing that the manufacturer of the bit that appears to have a bicycle chain design says it's able to supple up a horses mouth, yet people here (the ones defending the chain bit) are saying the opposite of that.
It is no different than our opinion on the chain bit.

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What doesn't kill you; is sure to leave a horrible scar. 
Success is not final, failure is not fatal: it is the courage to continue that counts.