...what I have found with AN4 bolts. The specification is 0.246 to 0.249 inches diameter.
If all of your AN4 bolts
really measure 0.250, then I suggest you talk to the procurement folks at Vans, since you are
not getting actual AN specification bolts.
I'm presently 6000 miles away from my hardware and can't measure them now...
... but I do know that I bought surplus 0.248 and 0.249 reamers for accurate holes.
..but as you say, ream the AN5 hole to 0.311 (or even 0.310 for a tighter drive fit) for the nose gear bolt in this particular appluication.
If you get good parts, the specifications work.
If the bolts don't meet specifications in diameter, then I would say that their entire pedigree and physical properties are suspect.
Examine you AN4 bolts and check their pedigree...