I'm building an Aero M5 and could to find a castle nut or back plate for the lower. Is it not required at all or depending on the butt stock? I plan to put a UBR on it.
Thank you.

Now that I had put the buffer tube on I see that it does not need nor have a place for one.

The M5 doesn't require the end plate to hold in the rear takedown detent (it's held in by the pistol grip). One still requires an end plate and a castle nut, however, even if you don't need it to keep the detent in. (Note that there's still a socket in the lower for the end plate.)

That said, the UBR has its own end plate and receiver extension that comes with it.

If you're installing the UBR on the M5, you're good to go with the hardware that comes with the UBR.

One thing I found I did need with the M5 receiver was a slightly longer magazine catch, so it would actually engage the mag button's threads. I had to get one from KAK. Strange, because no other AR10 I've built has needed a special mag catch.

Coy all OAT. The set and parts its are all Aero Precision so everything fits Rey well.

Not sure I'll do with it when done Exeter have to stockpile ammo for it but wel see!