Check the gas port in the barrel, it should be .093 or 3/32" or a rifle gas 20".
Some cheap ammo is known to be weak. If Winchester White box doesn't cycle there is a problem. If you want to use weaker ammo like 223 then you may need to open the gas port to .096 or .098. Use wire sized drill bits from a machine shop supply like Travers, MCS, Grainger etc.
The 6.8 is the fastest growing cartridge and #1 choice for hunting deer and hogs with an AR15.
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