Gas key is tight and staked, gas port in barrel is the correct size, gas block is adjusted properly, pretty sure it’s the magazine…Start with FACTORY ammo. Gas should lock back on empty mag. Mag can be a problem, try another. Check the spring length/buffer weight/full travel manually. Check BCG to tube fit/leakage. Check GB alignment. If good, next larger drill size for the barrel port. Stick a dowel in the bore down to the port. Turn drill SLOW.
'New build, follow the safety and operating checks for the lower.' Yup but nothing to do with cycling.