The Unibomber case was NOT solved by the FBI or LE. It was solved when a suspicious brother recognized Kosinski's writing style and dropped a dime.
Someone may be suspicious of a relative or friend, and if they hear "it was a .270," and that suspected person is known to have a .270, they finally call the cops and solve the case, Unibomber-style.
Why they are keeping the caliber secret makes NO sense!
I remember hearing a reporter say that they won't release the caliber because the type of bullets found could be used with multiple guns. Also, I remember early in the investigation the reporter that 'slipped' and said it was a 50 caliber.