Endshake washers do not address excess cylinder to barrel gap. In fact, if incorrectly used, they can cause cylinder to barrel binding by removing all clearance. One of their other functions is not often stated. That is to eliminate variable headspace. Since most revolvers headspace off the rim it can affect accuracy if the cylinder floats back and forth too much. I like to check headspace first to ensure that it is at minimum within spec and preferrably close to the minimum end of spec. After that is verified and corrected, then and only then do I set the barrel back. Basically build the correct relationships from the fixed breechface forward. So the cylinder gets any necessary shims at this point, before dealing with the barrel.
Setting a barrel back is a relatively simple process for a real gunsmith or a somewhat skilled amateur with access to a few tools and a lathe. Lacking a lathe, Brownell's did sell a barrel shoulder cutting tool (
076-200-319_Barrel_Set-Back_Fixture.pdf (brownells.com) made for the home smith to set back barrels that I believe is available to rent from 4D Reamer Rentals
The best chamber reamer rentals and gunsmithing tools from 4D rentals along with the other necessary tools like a receiver wrench, barrel vice blocks, a barrel facing tool, a forcing cone reamer, and a forcing cone gauge. Setting back one turn does not require doing any threading as there is more than enough left to get the job done.
Revolver wrench -
Revolver Wrench (Frame) - 4D Reamer Rentals (4drentals.com)
K frame wrench insert -
S&W 'K' Frame, MGW Revolver Wrench Insert - 4D Reamer Rentals (4drentals.com)
Barrel set back fixture -
Barrel Set-Back Fixture, Revolver (Brownells™) - 4D Reamer Rentals (4drentals.com)
Forcing cone and barrel breech cutters -
Revolver Forcing Cone Tools 38/357/9mm (11 degree) - 4D Reamer Rentals (4drentals.com). Note - available in 5* and 22* as well.
Forcing cone gauges -
Plug Gauges for Forcing Cones in Revolver Barrels - 4D Reamer Rentals (4drentals.com)
Basic steps are:
1. Clamp the barrel in barrel vice blocks,
2. Remove the barrel pin at the top front of the receiver with the proper cup tip punch (you can make one easily if you cannot find one for sale or rent),
Note: Newer S&Ws do not use a barrel pin and the barrels are installed at a higher torque.
3. Clamp receiver wrench around receiver just behind the barrel, ensure that it is snug and in full contact, but not too tight.
4. Turn the receiver off the barrel,
5. Cut the barrel shoulder in the lathe or with the Brownell's barrel setback tool (K frame thread pitch is 36 TPI so one full turn is .0277", stop a bit shy of that particularly if the barrel was loose or was overtorqued past TDC),
6. Reinstall in the receiver and reinstall the barrel pin (trial fitting and adjusting the shoulder so the barrel torques up correctly will be needed before final barrel install is complete),
7. Attempt to install the cylinder and yoke (probably won't fit),
8. Use barrel facing tool to shorten breech end of barrel about half the estimated amount needed,
9. Repeat steps 7 and 8 until the cylinder will just barely close,
10. Measure barrel to cylinder gap and continue to adjust the barrel face until proper gap (about .005") is reached.
11. Measure and cut the forcing cone with the gauges and cutter.
12. Go shooting!
That's a bit oversimplified and the devil is in the details. Anyone with good basic skill and an understanding of how this stuff works can do it. Patience is key.