continuing the polymer receiver primer:
There's two basic kinds. There's the newer gen New Frontier (NF) model, which is just the receiver, and there's the old gen been around a while Calvary Arms (CA) model, which is the receiver, buttstock, and pistol grip all in one.
Calvary Arms:
New Frontier:
Now the reality is, most parts of the lower receiver DON'T need to be very strong. The lower doesn't see a lot of high stress. The CA receiver has been torture tested, and held up to a lot of abuse. It's also non-customizable, and ugly as hell.
On the NF lower, my only point of concern is the stock joint, where you screw the stock in. That is also the point where recoil spring and buffer goes into the stock, and it does take a bit of abuse. I would hope that would not be liable to cracking way down the road. Please understand, I'm not well educated on the subject, I have NOT done my research, I am not a gunsmith, plastics expert, or machinist. All I'm saying is, you might want to read up on it first, that MIGHT be a point of concern.