One of the primary categories of Shabbat prohibitions (Mishnah on Shabbat 73a) is against “building.” Door bolts were generally unattached items, pulled from its socket when not in use. Now if it were tied to the doorframe with a chord, it would be considered still attached and therefore inserting is not problem. But if it is loose, it would not be allowed to insert it – not because of carrying but because putting these disparate items together is considered “building.”
Interesting to note that items which have other uses, like door bolt which has a handle and can therefore be used for other things, like a mallet, can be handled on Shabbat. It is a “vessel.” This includes such activities that you might think would be considered “work.” For example:
At the house of R. Pedath they had a beam which ten men had to lift to fix it in position at the door, but he told them no word against this. “It has,” he observed, “the character of a vessel.”
Since the beam could be used as a bench, it has a different purpose and is therefore a “vessel” which can be handled on Shabbat.
Lifting a huge beam in place to bar a door (an act which requires ten men!) may be permissible on Shabbat . But lifting a tiny unattached bolt is not.