and that is gas flow and pressure in a suppressed vs. a non-suppressed firearm.
A 6920 without suppressor functions well without it. With one mounted--and no further modifications--you will batter and beat the inner parts very badly, and parts life will be limited, indeed.
Solution: There are two. Either have a adjustable gas block mounted, or buy a purpose-built rifle with the proper sized gas port to function normally with the suppressor.
The best example I have seen can be found at Specialized Armament Warehouse. The owner is a gent by the name of Ken Elmore, who I believe is one of the single most knowledgeable people about the AR15 platform alive--other than the inventors themselves.
I highly recommend giving him a call, and discussing your options with him. If you decide to buy one, you will be doing business with Ken, who will manufacture your firearm, and (after all the paperwork is done) will transfer it to you through your FFL.
I have handled the XS6944 that he makes--and let me tell you, it is a SWEET little rifle. As a matter of fact, all other factors considered, you can buy the complete package from him for LESS than it will cost to buy a quality AR15 and suppressor separately. Added bonus--Ken's customer service and support are world-class. Take a look at the services offered: