Interesting project from Randizzle88 from Remington 870 Forum:
Well hey guys I found out for the 870 express 6+1 tactical shotgun, the Remington 4+1 barrels will not work. For some of you guys looking to fit a vented barrel on a 870 express 6+1. You will find nobody makes one where the mounting point is on the 6+1 870s. So here is what I did to make it work. I bought a 28 inch vent rib barrel 4+1 and made a spacer tube to fit a 6+1. I used a chop saw to cut multipurpose 6061 aluminum tube I got from the hardware store 1-1/4″ OD. Make sure it cuts a perfect 90 and slow cuts. Then I hand sanded the inside of the aluminum pipe to fit the magazine tube perfect.
Now take it to the sander and deburring the edges and cleans up the ends. Final take some scotch bright to the inside and out to get ready for cerakote. Make sure everything is clean before you clean out for cerakote.