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No, from magpuls own description "Optimized for use only with 7.62x51 NATO/.308 Winchester cartridges. Use with other calibers such as .260 Remington or 6.5 Creedmoor is not recommended."
As can be seen here under the description tab
natchezss.com/magpul-pmag-50-gen-m3-magazine-7-62x51mm-lr-sr-50-rds-black.html

NO, Magpul says the D50 is designed exclusively for 7.62×51 NATO and .308 Winchester. However, 6.5 Creedmoor and 260 Remington are not advised to be used in D50. I hope this helps

Gregory, yes Sir on the AR Pattern Rifle. I would check with Magpul to see if they have done any reliability testing on a 6.5 AR Rifle at 877-4MAGPUL (877-462-4785) 10:00-6:00 CST, M-F or by online inquiry here : https://magpul.com/about-magpul/contact-us