I have a model 19-3 6". I would like a 4" barrel for woods carry. If I were to fine a nice one on the open market, is it something that has a chance of being replaced without machine work done? Surely in the 70's with CNC machining, the factory didn't hand fit the barrel thread length to a cylinder, or clocked properly. What think you?

Barrel replacement remains, even now, a job for a very experienced gunsmith. It is NOT a matter of taking out the pin, unscrewing the old one, and screwing in the new one. Attempting that has ruined many frames over the years. My vote: send it to S&W.

Or, you could shorten the existing barrel.

And what Shawn said. Just because CNC machinery existed in the 1970's doesn't mean every manufacturer switched over to that stuff right then. It was massively expensive, required you to train up CNC service and programming techs and would disrupt your production while you were converting over. Not to mention the teething problems getting it working correctly. I was working as a machinist in the late 1970's early 1980's and ,especially in the firearms industry, a whole lot of machinery dated from WWII.

Also, one of the things you're talking about is thread timing. There's simply no way (that I know of) to establish exactly where on a part (so far as rotation is concerned) that the thread is going to start. You leave the threaded shank a bit long and fit it.

Several years ago I picked used M-15 at a gunshow priced right. The first time I shot it, it locked up with a cracked throat. I went to a local gunsmith that had an extensive parts room. I bought a used M-15 barrel for $20.00. Went home fit a 2 x 2 to the cylinder cut out. turned the old barrel off and installed the used barrel. Timed perfectly. The only problem was the cylinder would not close because the throat portion extend a couple thousands into the cylinder area. I took a flat file and feeler gauge and fit the used barrel throat to the cylinder. Shot great.
SWCA 892

If you want a 4" for carry, you'd be better off selling your 6" and buying one.

This is the answer. Nowadays, a 6" 19-3 is worth too much to jack around with it....it has good value to trade or buy/sell to get there.

True stuff! I started my apprenticeship in 1969 and after time laid off, finished in late 1975. We had one NC machine, and it was very basic (even by the standards of the time), and rarely used because not many knew how to run it, let alone run it well. GM, a bit larger corporation than S&W, was moving into NC machining, but at a fairly slow pace. Machines in use not only went back to WWII days, but well beyond. We had a planer that went back to somewhere in the late teens or early twenties. Unless a machine could verifiably reduce cutting time enough to justify cost, it didn't get replaced until something wore out, and a reasonably maintained machine could go for decades, even in a high usage scenario like ours, where some were run 24-7 for months at a time without ever being shut off. Nor can the NC (numerical controlled) machines of the 70's be compared to the CNC (Computer Numerical Control) machines of today. It's much like comparing an auto from 1970 to a 2018 model.

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So many S&W's, so few funds!!

These days, considering the fact that the prices for these older S&W revolvers continue to climb, I think you would be best served by selling the 6 inch Model 19-3 and buy a revolver that has your desired 4 inch barrel.

If I wanted a 4", I would buy one, and maybe I will. I would never sell the 6". I bought it new at a gun show in Sacramento in 1972 or 1973. $125 out the door. It has been the grandfather of my collection ever since. I was just wondering it it was feasible to change a barrel easily. I now hear from the experts NOT. I'm OK with that.

It's really not that hard to change.

1] A frame insert and barrel wrench are highly recommended.

2] A lathe if the barrel needs clocking is pretty much a must.

3] And a little basic math to determine how much to face off the barrel shoulder for the sight to clock straight up.

A barrel thread with 24 threads per inch is .0417" of barrel length per thread, one complete turn of the barrel. 1" divided by 24 = .0417".

Let’s say it’s 1/8 turn shy of clocking straight up, you need to remove material to get it to turn 1/8 turn more to be straight up. A 1/8 turn is 1/8 of .0417" or .0052" that must be’ faced off’ from the barrel shoulder in a lathe.

If the barrel turns in 1/8 of a turn farther than straight up, 7/8 of .0417" must be faced off for the barrel to turn 7/8 of another revolution.

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Jim
S&WCA #819

While the methodology is good, S&W uses 36 tpi, figure 0.0277" per turn. Also, most available frame inserts/wrenches don't properly support the frame. They are marginally better than the hammer handle through the frame opening.

It ain't rocket surgery, but it is persnickity to do correctly.