The G36c and the P90 have the same 247mm-ish barrel...
M4's have a barrel in the 370-400mm range.
With a M120 in a M4 you end up with high390-420fps depending on a bunch of different factors...
Same setup with a 247mm barrel nets you just around 350fps...
So, it sounds like it's working just fine.
It's been a while since I ran those numbers in different builds...so I might be off a bit. Short story is...your short barrel is the cause of a lower than expected FPS.
It's not the size...it's how well you use it...
