Max, as I posted on one of your other threads: Any barrel with a small ring will fit your gun, except for a special field barrel. Look for barrels which a quarter will not pass through the magazine ring. Since your gun is an express you should be able to find many barrels. Look at ebay.
The term "standard barrel" is used to designate a large ring barrel. Which is for the twenty gauge guns built on the twelve gauge receiver. So don't ask for standard barrel.
The Special Field was a short barrel english stocked gun and it has a magazine ring which is closer to the chamber. This barrel only fits the special field gun. IT was 20 inches if I recall correctly.