AFAIK, the current Abrams tanks (both in use and in storage depots) are equipped with (super secret) 'Chobham' type armor that incorporates depleted uranium. The US has laws against export of top secret gear, and modifying DU armor sounds both hazardous to workers and slow. Probably faster to produce some from 'scratch'. Yes, we could change the law - but have you seen our government? Our founders deliberately built it to be unwieldy, and now that there's been a 'big sort' (elected reps w no reason to compromise) it's gotten much worse.