Not sure if this will help as I don't have a HFT 500. In an S400 you need a 1.5mm allen key.

Hi, I also have a HW100 and a HW97K in .22, and now I have 2 S400's in .177. I find that I make sure that the pellet is the right way round then just place on in the tray and roll it in to the channel. I am now confident that I will load it correctly. You probably just need more practice. Regards Dave