MG, can you provide the links to these sites that are offering 'drop in's".
i jut guessing, but what i think they are meaning to say, is that thier chokes, will match the thread count and pitch of your barrel,
so if you have a Remington, then they will send you the choke with the correct threading...
i dont see how it would be possible to literally drop in a choke into a threaded barrel end and have it work. without some form of attachment, and without modification to the factory barrel threading.