Yes, the Precision Reflex M84 Gas Buster Charging Handle will work in a 300 Blackout.
No, This is specifically M16 and AR15 but we do carry charging handles made specifically for AR10 platform, for recommendations you can email us at techsales@opticsplanet.com and we can provide some recommendations.
Yes. It is possible.
Yes this is compatible with AM-15
Short answer is no. This is specifically M16 and AR15. They do make one for the DPMSLR308 and the DPMS G2 LR 308. Make sure you know which model you have. I have one on my DPMS G2 LR308 and love it. You should be fine and they were great.