The buffer removes much of the impact forces associated with the slide movement (battering) during the blowback action when firing. Without the buffer, or with a ineffective/worn out buffer the slide and frame will take undue stess. The result of firing with a defective buffer can cause malfuntions and eventually crack the frame or cause other damage. The buffer is a type of polymer (plastic) that deteriorates due to use and/or age. The supply of replacement buffers has been very hit and miss over the past few years but the good news is that Numrich currently has them in stock, can sell in lots of 5 if desired, and are pretty cheap. When bad they become discolored, brittle, and cracked. If you have any doubt about the condition of your HK4's buffer just go ahead and replace it.