Oneeyeopn, Your right the front cap is a triangle,but thats ok because a rifle length forend is made for a triangle.Carbines have a round cap. I wish I could post pictures of what mods I did. Which was basicly working down the tabs that go into/under the front cap. They are a little too thick and too long.The other thing that had to be done is to remove the sides of the aluminum heat shield because they expand when the lower handguard is put on the dcm tube causing the top not to fit on it.The best way I can describe the heat shield is a U with square corners at the bottom which thats where I cut the sides out.That way you still have the aluminum reinforcing the bottom of the hanguard for bipod mount and such.This has all bin done on a 20" barreled upper but I don't know if it will work on a carbine barrel because of the gas tube length if using a DCM free float tube.Using any
other type of rifle length float tube is possible but you will have to replace the front sight base with a low profile gas block and if you want iron sights thats not an option. Anyhow thats how I did it.
Good Luck 1HAWKEYE