[Kos-Mos]

I don't have an M14 yet but since I plan on getting one really soon I did some research.

I know that G&G and CA uses a different system than others.

I also know that TM and CYMA are 100% compatible. Most Echo1 AEGs are built on a TM base, so I would guess that you assumed Modify parts where built on Echo1 because of that. They are built on TM standard actually.

The Echo1 M14 is not built on a TM. They did a "custom" mechbox to save some money and reuse components.

Gears are regular V2/3 gears.
Piston is regular V2/3/6.
Piston head is regular V2.
Cylinder is regular AEG (V2/3/6).
Spring guide apparently is either V7 or V3 (I got mixed result in my quest for information).

Barrel is a custom one. Outer diameter is smaller and the cuts are not any standard.
Hop-up and rubber is custom, it is similar to a VSR-10/TM pistol rubber, but longer.
Nozzle is also custom. I haven't found any sign that anything else than the stock one will fit.

Finally, parts that are labeled for Kart, A&K, AGM and Echo1 are all supposed to be compatible.

Cyma uses TM parts.

G&G and CA use their own part too.

G&P makes a reinforced mechbox for the TM standard.

As for magazines, Star, TM, Echo1, A&K, Cyma all use the same one.
Apparently Star SCAR magazines work also, but I would stick to M14 labeled ones.

G&P and CA use special ones (they have a cut at the back and only work with their M14s)