BFAx

I'd never seen a JP in DPMS pattern they only make their own AR10 or Armalite pattern receivers for large frame receivers now and have for a few years as far as I know I have never bought a large frame JP but the 9mm line and AR15 parts are used in many of the rifles I build for myself and others . I was an 18B in the service and kinda took up this as a hobby after I retired . PS the video is from 2014

So I know this is old but just a comment unless I'm mistaken the OP says he's using a JP side charging upper with this JMT poly lower . The JP is an Armalite pattern /AR10 upper and the JMT is a DMPS Gen1 or LR .308 lower which is why it doesn't fit also likely why it broke when he tried to use it since it was engineered to work with a different rifle . PS as someone who's built a lot of rifles just my guess but even an aluminum lower would also fail at the buffer if you milled it to fit the wrong style upper .