Hey OM:

Brass is the old way to lower inductance. I like to use bismuth the
best metal for lowering inductance of a coil. There's no way that a
low power signal can cut thru bismuth.
And you can increase range by using tapered cone shaped slugs, and
centripetal casting is a lead pipe cinch with bismuth that melts at
271 degrees C.

I got a hold of sum 10 gram bismuth cylindrical slugs for like a buck
a piece. Really sweet for PTO and RF front ends. Depending on the size
of coil you have to match the right N150 to N750 capacitor, to
compensate for inductive temp drift. Or stick it in a small temp
controlled oven.
Or the new age method use AFC or PLL to catch the drift.

73 OM
de n8zu