I use that kind of set up(vert grip on rail), and as long as you tighten the bolts enough, they should be good. though i have no experience with M203s like that, im sure it would work as long as you are careful. or you could buy the kit for the M16/M203 combo. heres a link
http://www.doubleedgeairsoft.com/pro...aeg.php?type=6, there are 2 kits at the bottom.