Originally Posted by EW1066
That rifle was designed from the ground up to be made of polymer, and that makes all the difference. You CANNOT (in most cases) simply make the same part in the same form factor using polymer and expect it to perform the same or better. Unless the part was made from inferior materials to begin with. I.E. too heavy/strong or too weak for the intended job. When someone redesigns the AR-15 lower with the intent to build it from polymers, it might be worth a look. Until then, I'll pass.
That sounds like the best advice yet.
Problems with pins getting loose? Build a sheet metal cage like the H&K G3 /91 uses.
Weakness in the take-down pins? Redesign the interface, perhaps even completely do away with the front pin so that when the rear pin is punched out the front just levers away.
Lack of strength in the receiver extension? Design a completely different one, if it's strong enough mfg's will make parts for it. No one offered accessories for Glock when they first came out, did they?
Don't expect a copy of an aluminum receiver built from polymer to perform as well as a receiver purposely designed to be build from polymer. Accept that there will be certain things that can't be switched from one to the other.