To "pull down" is one way of "destroying" ammo.
The reason that ammo gets "pulled down" is that the ammo owner
(i.e. the gov or the ammo mfg [i.e. speer, rem, etc.])
wants the ammo to be destroyed for whatever reason.
The reason may be that that lot of ammo was found to be defective or out of spec,
or that lot or group of ammo was "surplus to its needs" and the owner wanted it destroyed.
Example: us gov decided that it didn't want to release loaded ammo in any way, shape, or form
[political policy] and/or the gov doesn't have firearms of that caliber anymore [i.e. 30 carbine, 30-06, etc.]).
Like many things the US gov does, it doesn't do it itself, it contracts it out.
Thus companies bid on providing the service of "destroying" the loaded ammo,
and sometimes the contract will allow the company to keep and sell the components.
Said company the provides a certificate that the ammo was destroyed per the contract.
Rational or not, if the gov or an ammo mfg allowed the loaded ammo to be sold
and either a gun blew up and someone got hurt, or someone shot someone with "gov surplus ammo",
some people would blame the gov and there would be a stink, and politicians don't like stink,
unless they are democrats and they are the ones throwing the stink.