The point at which the action bar meets the standing breech is the point of maximum stress under firing. The action bar must resist a bending moment because of forces acting upon the standing breech and the hinge pin (and the draw, if it's fitted to touch the circle). In an O/U this bending moment does not exist because the action walls act as a shear web.

Here is an FEA analysis that Boxall & Edmiston did: [Linked Image from boxallandedmiston.co.uk]

Last edited by Kip; 10/05/23 07:24 PM. Reason: Fixing image