For a gun-type bomb, 60% isn't enough. 85% is required. And as enrichment gets harder the higher you go, 60% is a lot further from 85% than it sounds.
Actually, once the enrichment process has begun it gets easier to enrich as the process continues, as the precursor U-235 is at a higher concentration making separation easier; the percentage of U-235 is 0.72% in Uranium ore. For those interested, this site is an incredibly good technical resource and history for all things nuclear weapon related:
https://nuclearweaponarchive.org/
Edited to add: Sorry archae86, didn't see the post above! We agree. Also, the Uranium gun type bomb was not tested, as its detonation was assured, and if the Iranians do enrich Uranium to the required level construction of a gun type device is trivial.