AITA: I Bought A MAC-11 Upper Receiver That Doesnt Fit.

I was excited to try out this AVES MAC-11 Upper Receiver, unfortunately it doesn't fit.

I tried to contact the company multiple times and they have never responded back. The website does not list the caliber of the upper receiver, so Im not sure if Im AITA...

Now I am stuck with a nearly $300 paper weight