A 6.5 Grendel AR15 style rifle will require you to swap out the bolt and extractor on your BCG for a 6.5mm bolt assembly like the WMD Guns NIB-X Bolt Assembly found using the link below:
https://www.opticsplanet.com/wmd-guns-nib-x-bolt-assembly.html?_iv_parent-caliber=6-dot-5mm-grendel&_iv_code=1UX-ABA-BA4N-1-NIBXBA65&_iv_caliber-193146313=6-dot-5mm-grendel