Don't know if the same is true for the SR-16 but I was able to get a non-RIS M203 onto my M4-S (RIS) by removing the bottom RIS cover. Just leave it off and see if the M203 will mount as it normally does (or better yet see if you can mount the M203 and then add the top RIS).
Image of M203 + M4-SIR setup -->
http://www.airsoftcanada.com/gallery...hp?i=2771&c=10