First suggestion would be a suppressor, seriously.
Flash hiders and comps do NOT increase the decibel level, they slightly alter the concussion to the shooter. In a closed room you would be significantly over the permanent damage SPL no matter what. Even with a suppressor, you will be over the damage thresholds. If there is a need, there is a better solution.
There are ways to cancel some concussion increasing effects to the shooter with some comp/FH features, but those are negligible and not enough. When SPLs are 140+dB, taking out 1 or 2 is hardly worth the effort. The Novekse Pig works to direct the concussion away from the shooter, but it pointed at a wall within say 10 feet or so, then the benefits goes away. The Battle comp creates multiple jets to try and disperse some, but again if those sounds waves hit walls, back it comes. The Dynamic Resistance comp uses some features to cancel out some pressure waves and remain neutral, and is probably the best universal comp. But still, none are going to be as good as hearing protection or suppression technology.