Link to PDF file with details on the recall. It is a smallish file.
Bushmaster Firearms Intentional, LLC has become aware of a possible firearms performance issue that may develop with a small number of ACR rifles and we are requesting you discontinue the use of this rifle immediately, and contact us at your earliest convenience so that we can make the necessary arrangements to have the rifle returned to us for inspection and update if necessary.Since the ACR has a firing pin return-spring, I don't think this is a slam-fire issue. I imagine it is a trigger group issue. The lower being an unserialized part, unlike an AR-15 lower, this would make things a little easier.
All Bushmaster produced ACR rifles are impacted by this notification.
UPDATE: It should make things easier, but they still want the whole rifle returned for the recall service.