A true M203 is 40mm (2.0 inches) and a nerf round is 2.5" You can make them work but they require modification. So do the shells that propell the rocket. I run one on occasion and personally I like the M203 but a lot of guys like the Metadyne launcher. It's 2.5" and requires no modification to get to work right...supplying it with air might be more of a pain depending on how you do it.
Here is a little something I wrote on using an airsoft style M203. The Phenom would take a R.I.S. mount M203 just fine.