That is correct. The notches for the hop-up require that you use a tight bore specific to the M14. You can modify a barrel to fit, but you must be precise in doing so. Just for fun I modified my stock m14 barrel to fit in a friends M4. All I used was a Dremel, and did it by hand. The M4 hop fit like a glove, although I'm not sure what he'll get out of it over his stock barrel.