There is no gun in Borderlands which fires grenades. The developers were going to have the gun originally but phased it out before final release. The files still exist in the game directory and this is why people who have access to weapon modification software can create such weapons. I do not recommend using such programs as they can ruin your save files and just plain ruin the gaming experience. "Modded" or modified weapons are becoming more and more popular, however they tend to ruin the multi-player experience in my opinion.