Disclosed herein is a modified rifle bolt carrier allowing a selectively openable vent/valve at the location where exhaust gas is pressurizing the bolt carrier to control carrier speed under suppressed fire in a first valve position or unsuppressed fire in a second valve position. A valve core is disclosed which may be rotated 180° to a first “open” setting for non-suppressed fire from its position in a “closed” position for suppressed fire, and a third “median” position. The modified bolt carrier will allow an operator of the firearm to adjust for a suppressor without changing the gas block or having to modify or adapt the front end or barrel end of the firearm.