Precision Reflex Industries (PRI) manufacture a 6.8 SPC magazine that get good reviews. A couple of forum members have used the 6.8 mag in the 6.5 Grendel without issue. I sent an email to PRI suggesting they introduce a magazine for the 6.5 Grendel/6 ARC. PRI advertise their 6.8 SPC magazine also functions with the 224 Valkyrie and 22 Nosler. PRI magazines are pricey but apparently good quality.
https://www.precisionreflex.com/Detail.aspx?PROD=186797
https://www.precisionreflex.com/Detail.aspx?PROD=186797