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.

Here's the Lucky 13 website LINK.

Here's a place called Cleaver Firearms Products selling them for $139, scroll most of the way down the page LINK.