What to use to crimp a resistor onto 18awg wire
A ferrule is NOT a reliable or permanent crimp device for your application.  The metal walls are far too thin.

Try it: crimp two resistors in a ferrule.  Bend them at right-angles to each other and try to spin them inside the ferrule.  You will find that they spin quite freely.

What DOES work reliably is the barrel from a standard non-insulated crimp terminal.  Find a crimp terminal that both wire fit inside and cut the terminal portion away from the barrel.  Put both wires inside the barrel and crimp with a proper crimp tool.

This makes a permanent, gas-tight connection that will last for decades.

FWIW - my preferred crimp style for non-insulated barrels of this type is known as a "W" crimp - so-called because of the "W" shape left in the barrel after crimping.  The AMP Certi-Lok tool with the "W" crimp die-set is one popular choice.

Almost as good is the T&amp;B "StaKon" crimp tool that electricians use - the small crimp nest works well for barrels up to 16 AWG, the large crimp nest works up to 10 AWG.

Finally, if you don't have any non-insulated crimp terminals handy, just remove the plastic insulation from insulated terminals.  The barrel is pretty much exactly the same as the non-insulated terminal.