Turned down shanks, and they bend easy as well. I agree a keyed chuck is the best way to go.
Glen is right the bits are turned down to 1/2 inch. Sorry, but you aren't going to get me to give up on the keyless chucks, I love them. I have 2 keyed chucks and I'm not going back. The problem, when it occurred, was not the bit turning in the chuck, but the chuck turning on the adapter. Not often at all. A little while ago a friend says he locktites his on and gives them a smack with a brass hammer and never has had them turn, so I'll try that.
The two new keyless that I'm going to order also have a spanner wrench that comes with them if you really want them tight on the bit.
Thanks for the tips, guys,
Sum