If you were making the barrel from scratch as opposed to buying off the shelf, yeah, it might blow up in your face. But that is the part prone to catastrophic f

If you were making the barrel from scratch as opposed to buying off the shelf, yeah, it might blow up in your face. But that is the part prone to catastrophic failure. People have been making lowers out of HDPE (plastic) and it works just fine, it doesn't need to contain combustion, just hold all the parts together.