What is a reduced diameter tube used for versus an original tube?
A big part of building a semiautomatic version of a full automatic firearm is making it so that it cannot be readily made to fire in full automatic mode. Of course, anybody CAN convert anything into full automatic given machining and welding skills and time, but the idea is to make it difficult, so that the home tinkerer cannot achieve it with hand tools and an hour or two of time. A reduced diameter receiver tube helps with this by not allowing a drop-in bolt from a full auto kit to fit. The bolt used for the build must be reduced in diameter to fit the undersized tube. So, if someone gets your home build and decides to convert it to full auto by simply getting a second kit and swapping some parts, namely the bolt, they will fail, as the bolt will not fit in your tube.
