I do not recall reading in any of Greener's writing that he considered the cross bolt as being needed to assist the underbolts in keeping the barrels closed. He did cite a case in which he constructed one of his guns with the crossbolt being removable by a knob on the end rather teing connected to the op lever. He fitted fixtures on the top rib & the breech so could stretch a thin non-elastic mat'l across the joint. He fired different loads both with & without the cross bolt inserted.
He stated his gun took all normal loads without rupturing the "Gage" even without the cross bolt but as he increased the load it began to break it. He went to still heavier loads with the bolt inserted & never broke it with the bolt being used.
The Doll's Head was of course designed for the same purpose. Many of them do not even have a bolt fitted so obviously do nothing to keep the gun closed. The original Westley Richards as do most Lefever Arms Co guns use only a bolted Doll's Head with no further bolts.
I have an old H grade Lefever "Parts Gun", the only one I have tried this on, but I can take the ball hinge completely put of the gun & reassemble it & it bolts up tight with only the doll's Head securing the breech & barrels together.
When a charge is fired there is definitely "Back Thrust". If there were not then any recoil or blow back autos simply would not work.
