Try this:
protected void onImpact(MovingObjectPosition p_70184_1_) {
if(!this.worldObj.isRemote) {
this.worldObj.createExplosion((Entity) null, this.posX, this.posY, this.posZ, 5.0f, true);
this.setDead();
}
}
You only need to create explosion in server side. The explosion data would be transported to client side via network.
The last parameter of createExplosion decides whether to destroy blocks. Set it false if you don't want blocks destroyed.