Do a search on Ebay for S&W barrel 19 and pick out the one you want. Unless you know what you are doing I recommend a GOOD REVOLVER SMITH do the switch
If you’re the adventurous type, you can easily do it yourself:
While a J-Frame is shown in the photo, the process is ALMOST * exactly the same for a K-Frame. FYI, K-Frame barrels are threaded .540” x 36 tpi, so make sure you’re getting an actual K-Frame barrel for your M19, because J-, L-, and N-Frame threads are different diameters, though all are 36 tpi. The barrel length doesn’t matter, as you can easily substitute proper length ejector and center rods to match.
The way I’ve done it, with uncounted S&Ws, is:
With a pinned barrel, first punch out the pin.
Remove the cylinder, but return the crane back into the frame. That’s to fill the space for wrench support.
Pad the jaws of a sturdy bench vice with leather or heavy nylon belt material so that no part of the barrel can touch the jaws during the process.
Tightly clamp the barrel into the padded vice jaws from the right (to remove a barrel), or from the left (to install a barrel), so you’ll be pulling the wrench toward you. Be sure the barrel is positioned so that the front sight does not bear against the pad at any time during the process, as it can be bent (don’t ask me how I learned that).
Place a thinner leather or nylon strap over the front of the frame, from the top, where the barrel is threaded into the frame. Make sure the strap extends down both sides enough that the crescent wrench jaws cannot directly contact either the frame or the crane.
Resist the urge to put a length of board or similar tool through the cylinder window to use as a lever or worse, using the rear of the grip frame, instead of using a proper wrench at the front of the frame. S&W frames can be irreparably warped that way.
Place an appropriately sized crescent wrench over the padded section of the frame, snug down its adjustment (I position the wrench so its solid jaw is toward me) and start pulling. The barrel will rotate some toward you until its under barrel lug (forward ejector rod locking pin housing) makes hard contact with the pad. Then pull hard. A pipe over the wrench handle can offer additional leverage, if needed. Once it’s broken loose, the wrench will no longer be necessary. For tightening and indexing a barrel being installed, everything is reversed. It’ll take some trial and error to get the barrel indexed with the front sight pointing perfectly straight up. Try very hard not to tighten a barrel much beyond top dead center, more than maybe a degree or two (I’m told it’s bad juju).
* IMPORTANT: On K-Frames and Model 63s, there is a flat on the bottom of the barrel at the forcing cone to clear the crane. So, don’t unscrew the barrel more than a few degrees with the wrench or you’ll damage the crane. Initially break the barrel free with the wrench, then remove the crane before turning the barrel any further.
There are some other issues concerning how much torque can be applied to S&W K- and J-Frame .38/.357 barrels when tightening, to avoid warping the interior of the bore. A S&W barrel’s shoulder should allow hand tightening until just a few degrees before top dead center (maybe the 11:55 position), otherwise it may take too much torque to complete the tightening.
Oh, this is how the above-pictured 640 looks now, with its modified 3” 60-4 barrel installation completed: