I use a Gemtech HVT on my 91/G3. It works fine in semi or select fire. I don't use subsonic ammo with it.
I have the same issue as BIGDUKE60, I wish that the mount covered the flutes as well. It uses the bi-lock mount flash hider (Gemtech uses the same bilock mount for HK 91 and 93 series - inner diameter is same as the 91 flash hider).
Not the best picture (cropped from a larger picture), but you can get a sense of how the hider looks.