Went looking for definition of .357 Registered Magnum. Found this in S&W Standard Catalog #3, page 133. "... This was the first S&W revolver to use a counterbored cylinder to protect the cartridge rim... " also have read this counterbored term in reference to barrels.

Would it be accurate to use a wood working term and think of this as a counter-sink precedure, i.e., having a hole already drilled in cylinder a larger drill bit is then used to make the hole a fraction larger and only for a small depth, allowing the rim of cartridge to fit flush or slightly below cylinder surface?

What useful purpose does the "counterbored barrel" have?

In woodworking terms, counterbored means a recess cut so that the head of a screw, (usually conical for a flathead) can be seated flush or below flush when installed.

In gunsmithing terms, counterbored means what you stated, a concentric cylindrical cut that allows flush seating of cartridge rims. This was innovative back in the early days of high-pressure handgun cartridges such as the .357 Magnum because the then-standard cartridge cases were all "balloon-head" construction, which was weaker than the now-standard solid-head cases. It is now commonly used only on rimfire revolvers.

Counterboring is also useful in restoring a barrel's muzzle crown when worn or damaged, as in many surplus military rifles that have seen hard service. It is standard on many competition quality target rifles.

A "Registered Magnum" is, literally a Model .357 Magnum of early production which was registered to the original purchaser. In the early days, from 1935 to just prior to the U.S. entering WWII this was done. The early guns were all special order items. There is a "Reg 1234" number on the frame in the yoke cut which corresponds to the Registration Certificate issued with the gun.

"Counterbore" generally has nothing to do with S&W revolvers except those with a two-piece barrel. In this case it has to do with the tooling used to install the barrels. With other firearms it can describe the style of the barrel crown, or a repair done with rifles with badly worn muzzles to remove the damaged section without actually shortening the barrel.

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Gunsmithing since 1961