4WG,
Only the standard will fit while using true M4 handguards.
However, I have a super extended length placed on top of my Colt M4 HGs... All I needed to do was dremel away a small amount of the HG for it to fit. It'll be ugly as sin if the rail is removed, but it works for me.
You'll have this problem with both of the extended lengths, but the standard will be fine.
Now, if you stick to the Bushmaster Shorty style HGs, you can use whatever you wish.
E-mail me direct if you have any questions.
Also, do a search for my name as a topic starter in this forum. I had a post with some pictures of my setup. The title had something to do with ARMS #38S-EX