My cat Yehudi is diabetic, so I have to give him insulin shots twice a day. The syringes are cheap (unlike the insulin itself). The vet gave me a prescription for them: I filled it at Target. The needles, I guess, would be small enough to represent 40mm barrels in 1/700, but my 63-year-old eyes can't see the openings in them - and I don't think many other people could either.
The 40mm Bofors barrel, if I remember right, was a straight, untapered tube except right at the muzzle. Every larger naval gun barrel I can think of has a taper that's quite noticeable (to my 63-year-old eyes) at 1/700. Hypo needles are hard steel; tapering them consistantly would be a challenge.
If you do decide to go the hypo route, the best way to get hold of a bunch of needles probably is to get on friendly terms with a doctor or vet. If that doesn't work, several products come in bottles with fine steel tubes. Check out the black squeeze bottles of Tests r's liquid cement. Or, if you have a decent hobby shop, see what it has in the way of applicators for model railroad lubricants.
Good luck.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.