Read all the tips related to Digger Launcher weapon on Gears of War 3 here.

The digger launcher is an incredible weapon when placed in the right hands. The weapon will go underneath the ground and will continue until it bumps into an enemy.

The gun has a huge range but is defeintely advisable to avoid shooting from long distance unless the enemies have no way of seeing the attack coming such as in this video. Notice how the screen blocks the vision of the digger approaching them.

 
The weapon works at close range as long as you ensure you dive backwards after using the weapon. The explosion radius is fairly small but anyone unfortunate enough to be in the path will have major troubles trying to avoid the weapon as it approaches.

Especially if you are in a narrow corridor such as the alleyway in which the weapon spawns on Old Town. This alleyway is a great place to protect and avoiding the digger launcher is almost impossible as the enemy has less angles at which they can successfully escape from.

continually using this weapon can amount to you aquiring many kills. Generally you will get at least one kill whilst using the weapon and any battles you encounter you are strong favourite to win as the active reloads are very quick and rapid fire can be achieved to a certain level. Be aware of the respawns of the weapon, if you have the weapon and you use all the ammo then it will take approximately 1 minute to respawn after you use the last of the ammo from it.  

Use this weapon on King of the Hill to deadly effect. Wait for all the enemys to enter the hill and fire a bullet into the air to land by their feet. This technique is great for hiding the fact that there is a digger heading in their direction. The less time it is in the ground the less likely it will be seen by the enemy. Aim high and go for the direct hit on your enemy.