I ordered the Lucky Thirteen magazine for the RARR from Black Arsenal in Stevensville, MI. Sent out a Postal Money Order and received it in a week. Great service.
This magazine has a machined aluminum top piece with a polymer bottom. Two flanges on the top fit outside the magazine well to stabilize it. Tested it with some badly battered soft point dummies that I had used to try and get the original magazine to work. Works like a champ. My RARR is in 300 Blackout.
To check it out, got to the Lucky 13 Precision website.
How about a link at Black Arsenal?
Searching for Lucky 13 Precision magazine and can't find anything.
They don't even list them under manufacturers.