I have a master JP dealer that I use, he has a GMR 15 lower on order for me. I should get it in about 2 weeks.
Since I live in CA. I shipped them a Thordsen stock to install, have one on my GMR13.
I too have issues with the GMR13 lower. I just finished putting the ODIN Extended mag release attachment on it.
Works great , part was only $22. Had a machine shop drill and tap it, reinstalled it and works great. Put the ugly
Taccom mag well on it. Can't believe how easy it was to install and reasonable priced, and works.
See if you can find a master JP dealer and order just the lower.