there is a very good article about cast bullets in 9mm Luger in the October 08 Handloader Magazine. You should take a look. Very often one may safely load and shoot bullets of slightly different diameter than the official caliber designation - BUT BE CAREFUL - start loads at the low end and work up carefully in small increments. In the article above the author discusses the diameters of cast bullets that have worked for him in 9mm - I think he likes .357 for some of his guns.
It is very common for cast bullet shooters to size their bullets .001 to .002 or occasionally .003 over bore diameter. Too skinny of LEAD bullets (.001 or more undersize) can cause leading and poor accuracy - more problems than .001 oversize. I once used up some lead 9mm (.356 nominal) bullets in my 38 Special (.357 - .358 nom) - they were very inaccurate and leaded badly.
Some variation is even acceptable for jacketed bullets - note some manufacturers make their 30-30 bullets .307 diameter (or used to) and some makers claim their 22 Hornet bullets are a tad smaller than the bullets they build for other 22 centerfires. If you were to precision mic any production bullets you'll find variation in diameter - some brands / bullet models tend to be smaller and some larger. Barrel bores vary as well - end to end as well as from barrel to barrel. I think that's why we see so many choices and what works well in one gun might not in another. Find a bullet that your barrel fits well and you get good accuracy. But as long as you start low and work up your loads, I would not sweat the difference (safety-wise) between a .355 and .356 bullet - they're likely designed for use in most 9mm caliber cartridges.
As a general principle, I approach 9mm loads carefully (but fearfully). It is a high pressure small capacity case and small changes can make big differences. Get several load manuals and you will probably see warnings like this as well as in the recommended loads. Some things to watch, seating depth, primer choice, amount of crimp, and of course powder wt. I'd add brass selection as well and recommend loading a single brand at a time, not mixed. Case capacities vary some and neck wall thickness can vary quite a bit. Consistent cases make for easier and more consistent taper crimping.
