The Forward assist on most DPMS type uppers is not like the AR 15/16/M4 rifles , it works , well some what works , like the last 1/8" it will hook up . I have dust covers on all my AR's & one of my 308AR's doesn't have a Forward assist ( Tactical Machining )& have no function problems . As said by some here before , if your 308 AR fails to feed , you have other problems & the Forward assist will not be much help .
What does "Assaults from the Beach have to do with anything ? The Dust cover is just that , a dust cover & I have never used the F/A on my 308 AR's , not much help any way.There are manufacturers making Bolt Carriers & Upper receivers to function just like there little brother AR 15 Family ( Areo ? )
I believe you should only get receivers as a set , Upper & Lower by the same manufacturer & any more , I only will use sets from manufacturers that make complete rifles also , so you know they have a good chance to work together .
I don't have the machinery available any more to do a 80% so can't help you there or how the finished 80% will function or co-function with other manufacturers Upper receivers.