First time poster on Firearmstalk forums. Hi @ all I am thinking about buying an AR-15 5.56 and I have some questions. 1) Is it legal for a civilian to have a barrel shorter than 16"? 2) Is it legal to hunt(Deer) with a small 5.56 cartridge? 3) Is it possible to swap the upper assembly from a 5.56 to a 7.62 without changing the lower assembly? My next set of questions are more technical. 4) What are the differences between 1x7, 1x8 and 1x9 twist barrel? 5) What are the general purposes of these different barrel types? 6) What gain cartridge should you use for these different barrel types? Thanks in advance!