Ok, so you know you want to be able to trim a resistor from 56k to 86k in order to adjust the speed of your rapidfire. On a trim pot there are 3 leads, the two outside leads are the ends of the potentiometer and you should be able to measure the full resistance of the pot accross those two leads. The third lead is the Wiper, this it part that if you measure from one of the outside leads to this lead the resistance will vary depending on what position the pot is in.
For what you want, you will only need to use 2 leads from the Pot, the Wiper and one of the outside leads. so in order to trim the pot to work from 56k-86k you will will need to start with a 56k resistor, so when the pot reads zero, the input will see 56k. So if you use a 30k pot connect the wiper to the input, choose one end to use and connect to your 56k resistor, and the other end of you 56k resistor will go to ground or wherever the other end of the 56k-86k belongs.
Hope that helps.
