i got the 0.692 and its great fits all paint
wouldn't recommend anything smaller than the 0.688 unless you get a few backs.
to Camdenelectric, the stock barrel's bore size wouldnt be too small as its a 0.695 or so im told
, mine chopped a lot, took it to my local shop and dave sorted it out for me while fixing a leak, think it was that i was missing an o ring or hadnt lubed it enough.