Bucky145

Not sure where my first attempt to post went.. I just wanted to stop by and say Hi. I'm new to shotguns and have the same questions that have most likely been posted thousands of times but I can not seem to find the answer. I would like to purchase a pistol gripped M4. It is my understanding to add the mag extension or the full length tube five parts need to be changed out with US made parts. Is this true? and what parts are most people changing out to stay legal? Where do I get these parts? I also looked at the M2 and was told by a FFL that I could add the mag extesion to it with no legal trouble. I thougt the same would be true for the M2 as the M4. Am I missing something? What about chokes? I know nothing about chokes. Does the M2 and M4 use chokes? If so, what do I use for 00 buck and slug? Thanks for putting up with the questions. I just do not want to run into legal trouble. Bucky145