Few know that Ruger resorted to machining a flat as well on some models:
"...At Ruger, revolvers were assembled with a proprietary lubricant similar to Militec to help prevent the lead from binding. Applying a few drops of Mil-L-63460B (Break Free CLP) in the crane arbor each time you clean also helps. Ruger developed a "hubbed cylinder" version of the Security Six, Speed Six and Service Six revolvers to mitigate the binding problem.
This required milling a small flat across the barrel extension, which protrudes into the frame opening at the 6:00 position, to clear the hub on the cylinder. Machining the flat reduces the cross section though the barrel extension, which caused heat cracking problems when those revolvers were shot extensively with .357 Magnum ammunition. The hubbed cylinder was used only for law enforcement contracts for revolvers to be fitted with .38 Special cylinders when the lead +P ammo was specified.
In designing the GP100 revolvers, the charge hole spacing, and distance from the bore to cylinder axis was increased so that the cylinder gas ring could be incorporated without reducing barrel wall thickness through the exposed forcing cone region..."
Ed Harris: Revisiting The Full Charge Wadcutter | Reloading, Ammunition, Hunting | GrantCunningham.com
"...At Ruger, revolvers were assembled with a proprietary lubricant similar to Militec to help prevent the lead from binding. Applying a few drops of Mil-L-63460B (Break Free CLP) in the crane arbor each time you clean also helps. Ruger developed a "hubbed cylinder" version of the Security Six, Speed Six and Service Six revolvers to mitigate the binding problem.
This required milling a small flat across the barrel extension, which protrudes into the frame opening at the 6:00 position, to clear the hub on the cylinder. Machining the flat reduces the cross section though the barrel extension, which caused heat cracking problems when those revolvers were shot extensively with .357 Magnum ammunition. The hubbed cylinder was used only for law enforcement contracts for revolvers to be fitted with .38 Special cylinders when the lead +P ammo was specified.
In designing the GP100 revolvers, the charge hole spacing, and distance from the bore to cylinder axis was increased so that the cylinder gas ring could be incorporated without reducing barrel wall thickness through the exposed forcing cone region..."
Ed Harris: Revisiting The Full Charge Wadcutter | Reloading, Ammunition, Hunting | GrantCunningham.com
