What are you planning on doing with it that you need it sturdy and heavy.
Aluminium is light but also very strong. It holds conservatory roofs up! Lol.
Regards
Almost all Airsoft M4 receivers generally only contain about 4% Aluminium. Most are made from a Zinc alloy called Zamak-4 as it's easy (and cheap) to cast.
Specna Edge bodies are very light - the Edge I have is lighter than my plastic-bodied guns of similar size. It implies that they either are thinner and have less material in places for structure, use a lesser grade of casting material, or a combination of both.
On their sales blurb they say this:
View attachment 71064
The only steel on the gun will be the body pins, dust cover, screws etc. The receiver, rail, outer barrel and buffer tube are likely to be Zinc Alloy.
So to OPs question.
Pretty much any Airsoft M4 receiver, unless explicitly stated, will be likely made of the same or similar materials to the Specna although the quality obviously varies between manufacturers. G&P, Guarder and Madbull sell aftermarket receivers, and they will likely be stronger and nicer than the Specna. If you look to vendors in Asia, you can pick up other manufacturers offerings such as King Arms.
But if you really want strong and sturdy, then RetroArms produce a receiver milled from a solid block of 7075 Aluminium. It's eye-wateringly expensive, It'll set you back far more than what the gun cost itself and then some.
I would just use the stock receiver until it breaks, it's not worth spending 60-70% of the gun's value on replacing the receiver if the original is unbroken. For what It would cost to replace the receiver, you could buy a lower-end Specna Core as a backup gun to keep in your kitbag should your main gun go down.
