One thing that has been said above, but that I want to emphasize, is to use the tailstock.
Gripping large stock in the reverse jaws means that the chuck has less of a grip (ergo, the stock has more stickout).
A few months back, I had used an annular cutter to make a 3 1/2" OD, 1.5" ID ring from a solid aluminum round. All went well until about halfway through parting the ring off. I was using an HSS T-style blade instead of my usual carbide insert, due to the depth. The friction from the parting operation caused the part to slowly creep out of the chuck - not enough to fling the stock anywhere, but enough to bind and snap the parting blade I've got a nice pic of it on my phone somewhere.
Lesson learned: always use a tailstock center or drill pad when working with those reversed chuck jaws if there's stickout.