my 16" LR308 upper is overgassed without being suppressed. i cant imagine trying to run it suppressed without an adjustable block. the biggest issue that peticular upper faces is a lock-time issue. it gouges the case rims pretty hard - nasty ejector and extractor swipes on the case head & rim.
i have a form 1 waiting at eforms right now, and i'm planning on needing some adjustables for my .308 uppers once they come in. i'm also getting ready to order a XHeavy buffer for it as well to help slow down the acceleration as well.