Of course the big difference between MT and ER collets is one is pulled in to the taper the other is pushed.

Pulling means you can't split the collet from both ends so trying to draw it below the stated size is going to put some serious stress on it.

To get the best from ER collets you first need to dump the crummy spanner that came with it and buy the multi tooth job. Gives you a lot more confidence tightening it when you know it can't slip off, more leverage to. Second is scrupulous hygiene, check the internal taper is clean and smooth before you fit it, zap the nose internal and the collet with shop air and look for stickies.

I did try the ball bearing nose but it seemed to make not one jot of difference.

If you are expecting a lot of vibration, give up and go for a Clarkson collet with a threaded cutter. You know it makes sense :tup: