You might need to remove the lock barrel if you have a problem with it or you are changing your locks. If so, this is how you do it.
I did this on a lock in a piece of steering column I got from a breaker’s yard.
You need to insert the key and turn it approx 55 to 60 degrees, I think this corresponds to position I (accessory). For safety it is probably better to disconnect the battery before you start.
Get a thin allen key or stiff piece of wire and insert it into the hole - the hole should now be “open” whereas it was closed in key position “0”. The depth of insertion is just about spot on 20mm.
As you can see from the picture, when the wire is inserted it retracts a small metal catch from the side of the barrel, enabling you to extract it by pulling on the key. You have to push the wire quite hard, and note that it goes in at a slight angle. You will see that I have taken the picture exactly edge-on to the key so you can see the shallow angle required.