What is a blocking rail?
As mentioned in the above question, wherein a semiauto build of a full auto gun cannot be readily convertible to full auto, some modifications will need to be made. Ideally, a reduced diameter tube would be used. However, this is not always the easiest option. This is where the blocking rail comes in. It is a round or square, or even flat but wide, key that is permanently welded inside a standard diameter tube, before it is final assembled, running along sufficient length to ensure that an original, unmodified bolt from a full auto kit cannot enter the receiver far enough to fire the cartridge in the chamber. Ideally, it is physically in the way of part of the full auto mechanism that was in the original design. The original bolt would have a slot cut along its length so that it will fit over the rail and move fully forward in the receiver. If you have the rail in the way, then the slot is cut through the important area, such as the original sear notch. This way, a full auto bolt canno
