maybe it means that if you are calm, you are rusty ^^ well, or they are the opposite of "restless"
(no "Wer rastet, der rostet" reference neccessarily intended, but rather because it sounds and reads like "rusty")
but yes, it contains "rest" (not as in "remnant", but.. pause)
and so "rustical" might want to indicate introvertedness
instead of again giving any unqualified comments such as "Die rostigen Jungens LaZeRn 'nen Bock."
yikes.. I just did, though -.-
(would be counterproductive if they rusted and also if they don't put the gun to non-vaporize, respectively if they caused the wool to regrow worse... weaker... and less)