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So lets say I had a shotgun, a Browniing BSS design with no barrel extension. What would a dolls head extension add to the design?

Thanks

Are we discussing the simple doll's head top rib extension?

Parker with Bernard I barrels, Bernard something rib and steel top rib extension



Or the top rib bolting system?

Charles Askins, The American Shotgun, 1921
"It remained for America to have the final word in bolting mechanisms. Lug bolts located on or as part of the barrels were in the wrong place. The hinge-joint of the barrels acts as a fulcrum of a lever, upon which, the barrels rest and pry at the bolts. It takes three times as much strength in bolts, to withstand the pressure if they are placed an inch from the joint, as it would should the fastening be accomplished three inches further away.
Alex Brown moved his locking bolts from the lug and placed them in the extension rib, which is undoubtedly the right position for them mechanically.
Throughout the world no other locking devise should be used on a shotgun save the rotary bolt."

Some form of primary or secondary top fastening system was used by Westley Richards, Greener, Rigby, and Beesley in England, Merkel and most German-Austrian makers, the Manufrance Ideal, and in the U.S. by L.C. Smith, D.M Lefever, William Baker designed Ithaca and later models, Baker and Hollenbeck designed Baker Gun & Forging guns, Remington 1893 and 1894, and A.H. Fox.



Ithaca also used an underbolt



Greener in The Gun and Its Development 1907 provides an historical perspective on bolting mechanisms.
Page 145 "The Westley Richards' Breech-Loader"
"This gun is one of the first, if not the first, of the top breech-bolt mechanisms, and was patented in 1862. In addition to the lump underneath the barrels, upon which they turn, here is a lump projecting from the breech ends at the top of the barrels. This lump is of dovetail shape and has a hook; the projection fits into a correspondingly shaped slot in the top of the standing breech, and is secured there by a holding-down bolt sliding to and fro in the line of the barrels.
This bolt is pushed forward by a spring behind it, and is withdrawn by pressing the lever lying between the hammers to the right. The object of this particular arrangement is to prevent the standing breech from springing back at the moment of firing, and was undoubtedly a step in the right direction.
In 1865 the author invented a top cross-bolt, which passed through an extension of the top rib, thus wedging the barrels to the standing breech."

Chapter 15 of Shotgun Technicana by Michael McIntosh and David Trevallion discusses "Top Fasteners" specifically the Alexander Brown patent, and the almost identical Ansley Fox patent.
"The most basic principle in fastening any hinge is that the farther the fastener is from the hinge itself, the greater the mechanical advantage. That’s why door latches are placed where they are. In a gun, the action bar and breech face form two sides of a right triangle, so that the top or the breech face is farther from the hinge pin, in straight-line distance, than the bottom. In a side-by-side gun
there’s an additional advantage in that a top fastener is a bit more efficient in overcoming the effects of barrel flip-for the gun, if not for the shooter. With a top fastener, the barrels still flip downward on firing, but the action bar flexes less, and the fastener keeps the top of the barrels from pulling away from the breech face."
"The most interesting of the American top fasteners is the L.C. Smith’s patented by Alexander Brown in 1883. Where all the others used either Greener’s transverse bolt, some sort of flat bolt or hook on the front end of the top lever, or both, Brown designed his bolt as a steel cylinder that turns on a horizontal axis. A slot filed into the cylinder forms the actual hook, which fits into a slot in the rib extension. The cylinder also engages a lip at the rear of the extension as a secondary bite. L.C. Smith described this arrangement as a double cross-bolt, although double rotary bolt would be more accurate."

As originally designed by Westley Richards the dolls head, and only the dolls head, kept the gun shut.

As in this circa 1880 Edwinson Green A&D still staying shut after 140 years.

On a strong modern gun such as your Browning BSS it would contribute absolutely nothing.

I think they help to reduce twist when the gun is fired. Also, if well fit, they keep the barrels centered when closing the action. Like guide pins do. less eccentric wear on cross pin.

One could argue that while Westley Richards guns were Birmingham made, many carried the 170 Bond Street London address, as in the case of these two examples. Unlike other and later uses of the doll's head by gunmakers after the patent expiry, these two Westley Richards guns have ONLY a doll's head attachment, no underbolt or other method, as the fastening system. One of the many believed flaws of the early breech-loaders was that the under-bolting systems used to that point were insufficient to keep the barrels from rotating on the hinge when fired, lifting the barrels at the breech and pushing the muzzles downward, thus making them shoot lower than the point of aim. The Dougall Lockfast (1860) used raised bosses on the standing breech fitted to the barrels to keep the barrels from moving upon firing, and was the first design to address this perceived 'problem,' to much acclaim at the time. The Westley Richards doll's head was one of the first attempts to have a snap-action that locked the barrels at the breech-face, or something close to it. A well-fitted doll's head was sufficient in and of itself.

Patent No. 2506 of 1862:


Patent No. 2623 of 1864, on a pin-fire conversion: