I have to echo kenpo. I am also concerned about how long they would hold up.
I can only imagine this was the mentality back in the 60's when the "aluminum" rifles came about. Back in the 90's I had one of the State Arms AR50 uppers. You would think that if any round would punish the lower, this would be the one. I shot over 100 rounds and I did not have any issues with the aluminum lower I used. As a matter of fact, I still own that lower and it now has a 16" pencil barrel on it. Talk about going to the extremes. I own a few poly lowers and will eventually build one into a complete rifle. I'll have no issues with this lower and it will shoot as well as anything else I own.
I promise...
Topped off with a 3X9 40mm. First AR build for an old C&R guy.
Link up with me in 10 years and many thousands of rounds, then we'll talk turkey with poly lowers.
Will you settle for 3 years and 10K rounds?
I run a polymer lower on my fun to shoot for shi*s and giggles 16" rifle and have had no issues with melting accuracy or separation of the DPMS upper from the lower.
So I think they have some promise like the polymer lowers on most modern pistols.
But that's another debate.
Also OP nice project. Looks like it'll run smooth. Please send a range report when you get a chance.
I can settle for 10k rounds over 3 years. Have you had any cracking or other issues with it?
How did you machine out the 80% lower? With a drill press?
No cracking or any performance issues I can think of. I even did a couple of mag burns just to see if it could hold up to the heat didn't have any issues.
The biggest gripe I had was that my adjustable stock didn't feel like it had a 1:1 match with the threads on the lower so I had to play with it to make it work.
The next step will be using it in competition. I think that will be the real stress test. Best part is if it breaks I'm only out $45.