Quoted:
What is the quality like and weight difference compared to an aluminum lower? Is there usually any modification needed to make it work?
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I have two friends who bought polymer lowers.
On the company's webpage, they had videos of them throwing a rifle around and then firing it (of course, with their company's polymer lower receiver). They did it multiple times, and it kept firing; no problems seemingly whatsoever. I tried talking both of them out of it, but those videos were enough for two of my friends to become convinced.
They both have had problems with their lowers since. One's lower fractured between the buffer retaining pin channel and the opening for the trigger group is. It fractured enough that when he pulled the upper off the lower, the retaining pin jumped out. He returned it to the company, who replaced it free of charge (except for the initial shipping). Still, who wants to have to keep returning lowers?
The other's front upper receiver retaining pin broke in half (because it was made of polymer too!). He bought a metal pin to replace it with. It wasn't a simple drop-in installation though; I had to cut a couple of coils off one end of the detent spring to make the metal pin fit in the polymer lower.
The moral of the story? Pay a few bucks more and buy a metal lower.