New Build, persistent jamming

[Zeneffect]

Sounds like shooting steel ammo, then brass.

She a dirty girl?

[Bonas]

Start from scratch. Disassemble the upper. Clean the chambers thoroughly. Find one of the barrels in which factory rounds drop freely from the chamber. Reassemble with the looser fitting upper. Ensure the gas block is aligned properly. Blow through the tube to insure no obstructions. Test the BCG in another upper. In short, make sure each part works right before moving on to the next step. You'll find your culprit along the way.