comparison chart of FF rails
My opinion of the more common rail systems
I would choose the DD Omega if I was not allowed to, or was unable to, remove the stock parts required for installation of the other rails.
I would choose the DD M4 rail if weight, simplicity, width, and internal diameter were critical for me.
I would choose the DD Lite rail if weight and a continuous top rail were critical for me.
I would choose the LT rail if price, overall height and an integral sling swivel were critical for me.

