It is also used in a few different guided missiles
Uh, yes they do.
It can hold super tight tolerances. Some of the tolerances I worked with were +/- 0.00002 on the beryllium missile parts. I suppose I should not say where they use it on ICBM's. I doubt it's much of a secret, but while it's interesting stuff, I'm not sure I want to go to jail.
You can find out why they use it in nuclear warheads by just reading published materials, but the other place they use it might be secret.