Can you use a BRN-180 lower with an MCX upper? This lower: https://www.brownells.com/rifl...rged-prod130438.aspx

Alternatively, can you use an MCX stock adapter on a standard AR lower with a BRN-180 upper?

This adapter: https://www.sigsauer.com/store/mcx-stock-adapter.html

I don't know the answer to your question, but it might be helpful to know that NoDak Spud also made a lower receiver for the BRN upper. If Brownells' version(s) don't work, it may be worth taking a look at the NoDak Spud.

Did you try checking the Q&A section on your link?
Because like....