Originally Posted by sarosh
from what ive seen, cqb m4's shoot below 350 fps.
No, a gun shoots whatever you want to make it shoot. A CQBR M4 is just shorter. You can have it shoot 500fps if you wanted to.
You mentioned Classic Army. That's a good start, and you may want to upgrade the spring like you said. An inner barrel upgrade may not provide much of an accuracy upgrade but that's your choice.