I'm not sure if you ever figured out the problem but i just recently ran into the same thing. When you insert the tube into the forend, look where the tube is making contact at it's end. Line up the end of the tube so it will go thru the forend. Kind of a trick because it looks like the rails are what is stopping it but it's really not, it's the tube end that stopping you.I tried to replace the forend but the Blackhawk one wouldn't fit on the slide all the way through. Ended up putting the stock forend back on. Has anyone else had a problem with the Blackhawk forend on an 870?