Rifled Nerf Rival Fate Barrel for Mega and Mega XL

This is a drop-in part that enables the fate to fire Mega and Mega XL darts accurately with no permanent modification. With the stock spring you should expect to get at least 75 fps with Mega and 45 fps with MXL, though a smoother print and/or looser fitting MXL darts seems to improve performance significantly beyond that.