1/2 x 28 TPI is the industry standard for M16, AR15 type rifles and also appropriate for custom applications. The .750 outside diameter is designed for standard military contour barrels, but can be tapered down for smaller contours. The JPRE-2 has a final exit hole of .281 and will accommodate bores up to .243/6mm. The final exit hole of the JPRE-2 can be enlarged to accommodate bores up to .358. For your 5.56 AR15, M16, M4.