I'm certainly no revolver expert, but from what I've gathered over years of reading gun rags in the john, a consistent gap is one part of the equation, but you also want to check for cylinder end play (the fore and aft wiggle) and alignment (how well the cylinders line up with the bore).

I would tend to think you're in the ballpark range with the .04mm - think of it this way - when a round goes off, the bullet has to jump the gap between the cylinder and the barrel. So the gap should probably be just enough to not impede rotation of the cylinder, and also aligned with the bore.

Dang, where's Bob Wright when you need him

Originally Posted by A_J

Dang, where's Bob Wright when you need him

Yes, I was thinking the same thing when I first saw this... Too bad our revolver expert is away....

Was searching google for "357 magnum long barrel" and this text came up on the first page....

The barrel/cylinder gap gauged at .004 inch, which is about .002 inch tighter than the average standard for .357 Magnum revolvers in general. ...

I have no idea of the source or the acuracy, but offer it for what it's worth!

The correct cylinder gap for a Smith & Wesson revolver should be .006 inch give or take a thousanth or two. If the gap is too large, velocity and accuracy suffer. If the gap is too tight, carbon and crud can build up causing the cylinder to bind or in severe cases the cylinder can even lock up.
Generally a cylinder gap of .004 to .008 inch is acceptable with .006 inch considered ideal.

Nothing over .0012 my 44 mag. mountain gun is a tighr .oo8. works great no leading around forceing cone.

On revolvers the cylinder gap is in spec between .002" to .006". Make sure on that old gun that it locks up and is in time. Pull the hammer back slowly and right before it's all the way back the cylinder should lock up. Then make sure it's ture to the barrell. That cylinder should not wobble on the crane either. If the rifling's OK and the price is right , buy it. Them old Smiths are hard to beat.