The bore was too large. Seen that. When gauged, the bore of the gas block mics out oversized or, worse, out if round when checked with a dial indicator. This seems to be more common with adjustable gas parts than with old school stuff. I have actually never seen it on a taper pinned FSB. My go-to-hell rifle still has the taper pinned FSB that fn put on the barrel but I cut it from 20 down to 16 and rebored the gas port. Shoots soft but is HIGHLY reliable. Not svelte but it works every time.Some manufacturers have oopsies in their products as well. The PSA adjustable block on my 6.5 grendel was absolute dogsh!t. It burped gas front and rear, no matter how tight the screws were. A trip back to PSA and a (we returned it to factory settings) didn't fix things. I tossed an aero adjustable block on and the gun immediately ran like a top and stopped burping gas out all around the block.