Below is a link to where this is discussed in greater depth, and I really recommend Vic Venters book "Gun Craft". He has a whole chapter on this subject and gives the best explanation of this that I have ever read. Also shows a way that this can be corrected.

Basically, the "circle" is the forward arc of the rear lump. In a double rifle, this should fit tightly against the mating portion in the action bar. This takes a lot of stress off the hinge pin and keeps the action from "stretching" open when the gun is fired. Generally, if the pressures are kept down in the 35K psi range or less, the shotgun action is usually okay, but as the pressures start go above that, the fitting of the circle is more critical. A loose fitting circle can adversely affect the cartridge cases as well, if the action flexes while shooting.

http://forums.accuratereloading.com/eve/forums/a/tpc/f/760101804/m/5861016271

Ellis