I have had my share of M4's and M16's.....I have used the "flat-tops" and the standard versions equaly and I will say this:
Not once have I used the carry handle on the M series. I use a sling.
An armorer had once said to me that the carry handle is for more the rear sights placement than anything else.
I am sure you have seen many war movies that show our heros running with an M16 and holding it by the carry handle. As a former Cav Scout with the US Army, NO ONE runs with thier weapon in that way. It is either in the ready possition, slung or in a semi-ready possition.
And in the game of airsoft, 99 % of the time you will se weapons slung or in use. Just get a good comabt sling and attach it to your web gear or tac vest.