I use polymer AR Mags in my 6.5 Grendal Ruger Predator...

one guy I know who shoots competitive at Military Rifle Matches, is a Retired Marine..
he had a 5 round mag that fit the Predator on the ARs he uses...

I preferred the 10 rounders, so I went over to Sportsmans and bought 5 of the polymer 10 rounds...

My Predator in 6.5 Grendal, was a jam-a-matic, but works beautifully with the after market polymer mags.

"Minus the killings, Washington has one of the lowest crime rates in the Country" Marion Barry, Mayor of Wash DC

“Owning guns is not a right. If it were a right, it would be in the Constitution.” ~Alexandria Ocasio Cortez