Yeah, these flat-topped Federal Arms G3/HK-91 receivers are crap in my opinion. If you add the prices of the individual parts, they shouldn't add up to $850. There are previous threads that cover this subject exactly. Try a search for "G3 Clone ", "Cheap HK", or Federal Arms.
Unfortunately, the only Hesse Arms HK receiver I've seen was no better. Some of the attachment points were visibly off from square and the welds looked like a trainee did them.
Either buy the real deal, assuming you have the money, or find something else that fits the bill.