basic dimensions for npt american national taper pipe threads
National pipe thread The taper rate for all NPT threads is 1 inch of diameter in 16 inches of length (3⁄4 inch per foot or 62.5 millimeters per meter) measured by the change of diameter (of the pipe thread) over distance of thread. The angle between the taper and the center axis of the pipe is tan−1(​1⁄32) = 1.7899° = 1° 47′ 24″.
Remington Model 11-87 The 11-87 incorporates a self-compensating gas system design, which allows the gun to operate with every type of load, from light 2 ¾" to 3" Magnum shells without any shooter adjustments. It is manufactured in two different gauges: 20 and 12, and both will cycle 2 ¾" and 3" shells.