Hiya,
Sadly that’s a very complex question as the answer changes from model to model and maker to maker. There are a hundreds of variations between them which I assume is a way of stopping conversion by adding “real” components to them
Short answer is that modelguns try to replicate the real counterpart as closely as possible in look, size, weight and function.
But….. with a lot of exceptions
Material
A move more to plastics and away from metal
Size
Some models have slightly (and not so slightly) different external/internal dimensions
Internal components
The internal components can vary in size, shape, function and placement to real internals.
Sorry to be so vague but I could type for hours about the differences between the MGC, Marushin and Hobbyfix AR15 family .. And that’s just one type of model, the same applies to 1911s, M9/92FS etc.
8Ace