It could be a combination of things, the gas port may be eroding letting to much gas enter the carrier increasing the carrier speed. It may also be the case sticking to the chamber wall as it heats up so these 2 things combined could cause the issue.
Has the angle of ejection changed as you have owned the rifle or while shooting it during a session? If the brass is ejecting from 1-3 then you may try an adjustable gas block, I recommend a steel as opposed to alum. or a SSS buffer spring to slow the carrier speed.
The 6.8 is the fastest growing cartridge and #1 choice for hunting deer and hogs with an AR15.
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