If you manually open the bolt, you do need to depress the button in front of the trigger guard, if you want the bolt to stay open.

The bolt will stay open after the last shot OR, after you dry-fire the gun.

 

if you remove that little button and the the spring, the gun will then stay open when you pull it back and will operate normally. The only thing that changes is that if you put in a second shell, you have to push the release button to put the second shell in. I change all of my guns to work like this, especially if you shoot games and have to open the gun often without shooting it.