They are assembled by hand and having taken time to correct this issue on my 625 I can state from experience that lining the barrel up isn't nearly as simple as many assume.
First there is the amount of torque used to tighten the barrel, I would estimate it's around 50 ft.lbs. Note, having assembled MANY Chevy Small Blocks when I was a pretty serious gearhead I can assure you that estimate is pretty close.
Second, the barrel has to be "captured" in a vice block that will hold it securely and not mark it up. So there really isn't much of the barrel visible to line up with the frame. In my case the Delrin vice block I made only left about 1/4 inch visible at the rear where it met the frame.
Third, the frame wrench I was using is fitted to the frame in the area of the cutout for the yoke, so there is only about 1/8 inch of the frame showing where in meets the barrel.
Finally, that frame wrench has a "handle" that is only about 12 inches long. Ever try to hit 50 ft.lbs. with a wrench that is that short? You have to be a real gorilla to get there and have control. I used a 20 inch of pipe as a cheater and the end result is that I'm darned near arms length from the area I need to see clearly to get that joint to line up.
Now for the end result of that work. The barrel is lined up dead nuts perfectly. AND my 625 shoots slightly left. Turns out that I should have over rotated the barrel slightly if I wanted it to shoot dead on with the rear sight perfectly centered. However I'm not going to do anything about that, once was enough.
BTW, before I started the barrel was over rotated enough I had to run the rear sight full left to get it zeroed with my preferred range load. With my preferred SD load it shot too far right to correct for. I also decided to do this myself because i wanted to lap the forcing cone at the same time to see if that did anything for improving accuracy. Truth is I cant shoot well enough with iron sights to know whether lapping the forcing cone did anything.
