On a "Swiss" machine you are using the stock as part of the support system during cutting. When you adjust the guide bushing it must be tight enough to prevent the bar from escaping away from the cutting tool. When your bar stock size is changing you must set the bushing to the bigger size to prevent the stock from getting jammed as it tries to pass through. Now when the smaller portion of the bar is in the bushing the stock will escape for the cut. The "rule of thumb" is 60% of the stock condition will very likely be found in your part, out of round, varying size... You will also get finish problems from this. So if your stock runs .001" out of round your part will be up to .0006" out of round. Not always but very likely. Remember "Garbage in, garbage out"
There are special collet systems to help prevent this by using an "adjustable" guide bushing. Citizen has one that work pretty well. Recent design changes have solved early problems.
These systems are expensive but, add up the cost of scrap, broken tools, down time missed deliveries and lost customers then decide if the cost is worth it.