The M4 (V2) is a kind of standard. The issue you have is the manufacturing, all manufacturers use a different tolerance so often you get a part that isn't quite right or it's super tight, it's the result of manufacturer A being a bit sloppy and B being a bit too tight.
Perfect example, TM Hi Capas, the aftermarket CNC alu slides are often more precise than the TM frames they're designed to fit to so you end up hand racking them for hours to get the perfect fit.
That's airsoft and the V2 platform has such a wide plethora of parts you will always find somethign that doesn't fit and usually 15 that do.
I'm sure theres a joke about workmanship in there which would be very borderline.... Obviously not if YOU built it though 😁