There's nothing wrong with that E1 G36 that prevents it from being a good gun. If internally it works well and gives a decent velocity, then that's all you need. For accuracy and range, install a tightbore barrel and new hopup rubber and bucking (like Guarder) and you're good to go.
As for the other guns, there's not much you can do to make them "formidable" that wouldn't cost more than buying a quality, brand name version of the same guns.
__________________
|