I have bought an SRC HK416 and I also own a G&G M4, both lower receivers are made of plastic but they are not similar at all. SRC lower receivers are very strong and slightly rugged to the touch, they are also curiously much more tinted than the usual G&G smoked lower receiver. From afar you can't even tell it is made of plastic and from up close are very dark. I also own an ICS MP5A4 and I can confirm that SRC upper receiver are much more resilient to scratches. There were also no wobble at all on the SRC HK416 which was welcome. Anyhow that's all I can say for the time being on my experience with SRC.
Here's a picture of the amazingly dark and sturdy lower receiver from SRC.