STOCK TUBE & STOCK DIAMETER
To find out if the stock or the stock tube will be compatible with your replica, you can check the diameter of the stock tube to determine if it is MILSPEC or COMSPEC.
For M4 AEGs, as a general rule stock tubes and stocks from Tokyo Marui - VFC - G&G and clone "M4 AEG" will be closer to mil-spec size.
For the M4 GBB and AR15 beware it often happens that the tubes are bigger so measure the diameter of your tube carefully before ordering. At Magpul, for example, all sticks are sorted MILSPEC or COMSPEC which makes your job easier!
Obviously the manufacturers all have more or less their own rating and it is unfortunately impossible for us to promise you that you will have the stock never moving on the stock tube !
Stock Commercial Or Mil-Spec version: