The end of the barrel was clipped off completely, and then replaced with a catachan lasgun barrel. If you don’t have any spare catachan lasguns sitting around, but are planning on building a few black mariahs, you can use the lasguns off of those sprues instead. This adds about 25 to 30% to the length of the gun.
Remember all those barrels you clipped off for the arbite shotguns? Well I hope you saved them, because now it is time to use them. Glue the barrels end to end and then place them on top of the gun for your scope.