A 20ga will have 2 1/2" chambers if still original.
Many have been lengthened to 2 3/4" for convenience of modern OTC ammo.
Not necessarily a good thing as the chamber pressure of most modern ammo these days excedes what the gun was made for at the time.
Keep the loads in the 8000psi and below chamber pressure range and use 2 1/2" shells if still short chambered and you'll have no issues.
Flues frames, especially the small bores in 16 and 20 are known to crack.
A verticle crack on the side(s) right at the junction of the standing breech straight down towards the base of the frame.
That is from the breech being pushed back during firing and the somewhat sharp angle left there between the two elements.
MAny of the Flues were made as light-weight as possible and the standing breech is very thin. The breech behind the bbl's is filed extremely thin to save weight and at times no thicker than 1/4" at it's thickest point.
These early Flues were offered in Damascus & Twist steel bbls as well as Fluid Steel bbls.
Some of the Damascus Bbl'd guns have over the yrs been polished and Rust Blued which has hidden the damascus pattern making them look like Steel bbls.
A very careful examination in bright sunlight or a very bright flashlight will usually bring out the Damascus pattern though. Often the pattern will still be visible onder the forend area of the bbl where the Rust bluing didn't take well or polishing wasn't very good.
Flues models have poor Butt Stock fit-up design that leaves the stock to frame loose on many. That leads to the stock cracking especially on the top edges just back of the frame on both sides.
They often have a crack there and/or repairs from the past.
There is no internal reinforcement bevel as most SxS stock have where the top tang screw pulls the lower tang and the frame itself together with the stock and wedges all three pieces tight as the screw is tightened.
The top tang screw on the Flues is mearly a short stubby Wood Screw that is usually stripped anyway and hold nearly nothing in place.
The rear tang screw is the only fastener that actually holds the frame, trigger plate and stock together and with no wedging action.
The Bottom Tang screw is also a very small dia one,,a #6 IIRC.
All this says the guns don't take recoil very well. Stick to soft loads again.
The choke markings are as pointed out on the Bbl Flats at the front edge.
0-Cyl
S-Skeet
1-IC
2-Mod
3-ImpMod
4-Full
That's what the bbl was when it left the Factory, many have been relieved and are now altered.
Check for loose top and bottom ribs.
Don't bother with the 'Hang the bbl set and tap it & listen for the church bell ring sound'.
That'll tell you nothing, unless one of the ribs is obviously looseand detached at the muzzle.
Better to rap on the top rib with your knuckle and listen for the unpleasent sound of a loose section. It'll make a clank sound as opposed to the dull thud of a sound soldered joint.
Even this won't expose very small gaps at times.
Examine the ribs where they meet the bbl very closely. Use a loop.
Press on the rib and watch the joint. You'll see if it moves. If there is any oil in there it'll push out however small the amt.
Carefully go up and down each side.
Any loose ribs, leave the gun for someone else unless you can re-set them yourself and also re-rust blue the bbl set.
It's expensive work and more than a Flues is worth unless a High Grade gun.
Any bbl 'repairs' obviously done with any sort of glue or epoxys,,same thing..walk away.
