A system for treating a vein, comprising: a syringe comprising a syringe plunger and a syringe flange having first and second wings extending radially in opposite directions; a dispenser gun for delivering a vein-occluding substance into the vein, the dispenser gun comprising: a control, a holding segment comprising a plurality of opposing flanges extending distally from the dispenser gun, a lumen configured to accept the syringe plunger, the lumen extending proximally through the holding segment, and a slot at a distal end of the holding segment configured to mate with the syringe flange of the syringe, the slot having a distal opening sized to accommodate the syringe flange, and an interior channel configured to allow rotation of the syringe wings within the interior channel, the interior channel having a stop surface to limit rotation of the syringe wings; and a catheter having a distal opening and a side wall, the catheter configured to be operably connected to the syringe, wherein the catheter is configured to advance distally across a treatment zone in the vein; wherein activation of the control results in the syringe plunger advancing distally in the central lumen when the slot at the distal end of the holding segment is mated with the syringe flange of the syringe, causing the syringe plunger to expel the vein-occluding substance into the catheter when the catheter is operably connected to the syringe, causing the catheter to deliver a bolus of between about 0.05mL and 3mL of the vein-occluding substance into the vein; and wherein the dispenser gun is configured to deliver a plurality of spaced-apart boluses of the vein-occluding substance. a syringe comprising a syringe plunger and a syringe flange having first and second wings extending radially in opposite directions; a dispenser gun for delivering a vein-occluding substance into the vein, the dispenser gun comprising: a control, a holding segment comprising a plurality of opposing flanges extending distally from the dispenser gun, a lumen configured to accept the syringe plunger, the lumen extending proximally through the holding segment, and a slot at a distal end of the holding segment configured to mate with the syringe flange of the syringe, the slot having a distal opening sized to accommodate the syringe flange, and an interior channel configured to allow rotation of the syringe wings within the interior channel, the interior channel having a stop surface to limit rotation of the syringe wings; and a catheter having a distal opening and a side wall, the catheter configured to be operably connected to the syringe, wherein the catheter is configured to advance distally across a treatment zone in the vein; wherein activation of the control results in the syringe plunger advancing distally in the central lumen when the slot at the distal end of the holding segment is mated with the syringe flange of the syringe, causing the syringe plunger to expel the vein-occluding substance into the catheter when the catheter is operably connected to the syringe, causing the catheter to deliver a bolus of between about 0.05mL and 3mL of the vein-occluding substance into the vein; and wherein the dispenser gun is configured to deliver a plurality of spaced-apart boluses of the vein-occluding substance.