If you want your speargun to last a long long time you should never slam the speargun shaft into your speargun. If you have to slam your shaft in hard, there is something wrong with your speargun. Here are a few things to pay attention to.
Make sure the safety is in the right position, some spearguns will not accept the shaft if the safety is on.
Make sure the line release is reset all the way.
Make sure there is nothing obstructing the area where the shaft goes into the speargun.
Make sure the shaft is in good shape. Some shafts with broken or bent ends will not go in smooth.
Make sure the trigger is not hung up.
Make sure the speargun is not full of salt crystals or corrosion. Sometimes an improperly rinsed speargun will build up salt crystals that start to bind up the moving parts.
Your speargun shaft should reload into the speargun with minimal force, nice and smooth.