I did a search here and found one reference to a "Flash Enhancer", and in the context it was used, I thought it was a joke. But I just discovered that it really does exist and Keep Shooting is selling them:
AR-15 / M-16 Flash Enhancer (for 223 Rifles) | Keepshooting
AR-15 / M-16 Flash Enhancer (for 223 Rifles) | Keepshooting