Yeah, I tried everything - even very little stick-out on the bar. So, I am putting it down to the expansion arbor not being stiff enough. But it's OK as the reamer did a really nice job in the end.You can get chatter with the boring bar if the speed is too high or you take too big or ironically too small a cut. There will be an optimum depth of cut and speed, you just have to find it in terms of chuck rotational speed and lead screw feed speed. Leave as little boring bar unsupported as you can get away with.
That's OK. If they just wake up and actually make it afterwards, all is good;-)It would be interesting to be a fly on the wall in the Mares boardroom when they realize that while asleep at the wheel they have missed the opportunity to create a new gun that they should have done themselves.
They do have to massage the numbers a tad, though. Like going to a smaller pumping barrel and possibly using smaller main o-rings at the front. The latter is something I will be trying out on the next cone.