I thought I would offer a few pictures of the process I used to cut and install a 6" barrel shroud purchased from Tacticool. I decided to cut the shroud just shy of the end cap rather than to remove the end cap.
Step 1 - remove flashhider and endcap
Step 2 - install shroud
Step 3 - wrap tape around shroud just shy of where end cap would be
Step 4 - trim off excess using appropriate saw
Step 5 - grind down to make smooth
Step 6 - use a metal file to finish
Step 7 - reinstall end cap
Step 8 - install shroud and tighten with torque wrench
Note - I did use a little loctite on the threads of the barrel.