The ASP WE-Tech M4 GBB Hopup Adjustment Tool is a neat flexible hex driver that easily fits inside your WE M4 gas rifle. No more awkward adjustments with an allen tool! To use, simple pull back your charging handle and insert 'stiffy knob' into your chamber area, and adjust your hopup. Very easy! When done, you can store the tool inside your M4's pistol grip for safe storage.The best tool to adjust the WE M4 Hopup with!