Slip Gear Piston Nylon String Stopper Problem

Hello everyone! So my team (Team Vectors) has made a slip gear piston. In order to stop the piston at a certain point we are using a high grade paracord string. This idea was working great but the problem is that it only works for a few shots. The knots we used to anchor the string so that it does not move is not working. It is slowly slipping so every shot the piston moves forward a little more. Does anyone have and tips as to how I can tie it off? We are currently just tying the ends of the string to the c channel upon which the piston is on. Is there a different knot should use so that it doesn’t slip? Thanks in advance for all the input guys!!

We also ran into issues with tying the string as it always loosened over time. So eventually we made a large knot with the ends of the string and ran it between a C-channel and thin smaller plate, then screwed the two plates together really tightly. Basically, we sandwiched the string between metal to prevent it from moving. It may not have been pretty, but it held up fine for a full day competition.

Also, when you use new string, make sure to tie it shorter than you need. I would recommend pulling the piston back a little bit more than the position you want it to be at, sandwich the string, then fire the puncher for a few minutes to allow the string to tension and settle properly.