How much does your buffer weigh?
It should be in the 5 1/2 oz range.
Does the bolt lock back on an empty magazine if you only shoot 1 round?
Do not confuse rifle length gas tube with the with what recoil system you need. It is all about reciprocating mass. BCG and recoil springs w/buffer have to work together with the gas pushing the BCG rearward.
Did you start test firing with the adjustable gas block wide open?
If it doesn't lock back on an empty magazine wide open your gas port is not large enough.
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