It doesn’t matter. They share a common hammer inlet.
Ruger Birdshead, Dragoon (SBH), Hunter, XR3, and XR3-RED all have the same hammer inlet, while the Bisley has a longer inlet, such 4 of the 5 Ruger hammer types, except the Bisley hammer, will fit in any current production (post 1998) Ruger grip frame. Bisley hammers must be modified to fit any non-Bisley grip frame by dressing down the radius of the “rump” of the hammer, according to whether you have a Pre or Post-1998 revolver. There are only 3 radii of Ruger centerfire frame hammers: largest radius is the Bisley, then Montado/New Vaquero, and smallest is the BH/SBH/Vaq. Without modification, Bisley hammers only fit Bisley grip frames, Montado and New Vaquero hammers only fit Post-1998 frames + any Bisley, and SBH/BH/Vaq hammers fit any non-3 screw frame ever made.
The upper screw bosses on Pre-1998 grip frames had a smaller radius than the post-1998 grip frames. Blackhawk/Vaquero and Super Blackhawk hammers still retain the smaller “rump radius” of the Pre-1998 era, which leaves a small gap between the screw bosses of Post-1998 Blackhawks, SBH’s, Hunters, and original Vaqueros. Only the New Vaquero Longhorn hammer and the Montado hammer have radii which match the Post-1998 grip frame curvature, filling the gap between the bosses. This also means these two hammers won’t fit into Pre-1998 grip frames, of any shape, without modification, same as the Bisley hammers.
Bisley hammers can be modified to fit either era of any non-Bisley grip frames, as depicted below (as can Montado and Longhorn hammers). Only two radii options exist for all non-Bisley frames of each era - Pre-1998 and Post-1998. Trim the butt down, they’ll drop right in.
At left is a Super Blackhawk hammer, beside it a Blackhawk/Vaquero hammer, and 3rd from left is a modified Bisley hammer made to fit a Pre-1998 non-Bisley grip frame. These will fit any Ruger centerfire mid or large frame ever made. At far right, a New Vaquero Longhorn hammer (Not pictured: a Montado hammer has the same radius as the Longhorn). Second from right is a modified Bisley hammer made to fit a Post-1998 non-Bisley grip frame. The two right hand hammers will fit and function in any Ruger mid or large frame revolver manufactured after 1998. The 3rd from right, with all of the “junk in the trunk,” is a non-modified Bisley hammer. Comparatively, the 3 on the left have the smallest “rumps,” the two on the right have a little more meat there, and the standard Bisley, well, ”Baby got back.”
View attachment 883536
Potentially also of note: all Ruger mid and large frame centerfire triggers also share a common sear geometry, however, the “shoulder” of the Bisley trigger is very different than the SBH/NV/Vaq/BH/Hunter trigger, such either the trigger or the grip frame must be modified when using a Bisley trigger with a non-Bisley frame. This task is much simpler than the hammer modification, which is really quite simple as well. To fit triggers, typically the grip frame is modified with a larger spur port to accommodate the longer trigger “body.”
@Mr. Mosin - if your buddy wants some help getting a Bisley hammer for his Birdshead or plow grip Ruger, for far less cost than the Power Custom option, shoot me a PM.