All M16 / M4 variants use a V2 mechbox. MP5s are grouped with the Armalites because they also use a V2 mechbox.
The only difference internally between the full-length rifle and carbines (or MP5s) are the shorter guns use a ported cylinder instead of a solid one. Otherwise, all internals and the mechbox shell are interchangeable.
You may also want to check
www.airsoftparts.ca for parts. They're in Canada (Toronto area), have very reasonable prices, and ship very quickly.