The Nail Gun

It's a Sandbox. There is no specific 'End'. There is no specific "I beat the game". There will eventually be a story, but even then, beating the story will not be the 'End' of the game; It will just be the end of the story and you keep playing after; Just like minecraft where beating the ender dragon isn't the end of the game, just the end of the 'story'.

Whatever goals you make for yourself to be an 'end point' is what your end point will be. If for you that means making a nailgun, that means making a nailgun. But it is still a sandbox, and the only way playtime in a sandbox ends is if you, yourself, decide to stop playing in the sandbox.