Hey guys, back from the dead. My computer (a.k.a. "Soul Brother #1") picked up a nasty virus from the Firing Line, so I have been down a while. (This was not that forum's fault by the way.)
You bet your sweet bippy ammo affects accuracy. The other side of the coin is that most handgun ammo is more accurate than the shooter! As others have noted there are quite a few variables at issue here, but it is mostly a trial and error process. I have not shot NEARLY enough rounds through my CZs to make any pronouncements, but here goes... (Some of this reflects reloading concerns, bear with me...)
1) My 75B Mil and 75B SA (both 9mm of course) like the same ammo. The SA is more accurate, but these guns have the same "tendencies" with the same ammo. I am a fairly new shooter, so this data may be skewed as well. I suspect that my bad habits make them more similar than the manufacturing process!
2) In general, Speer, Starline, Winchester, Norma, and Lapua brass seem to be more consistent, last longer, and have more uniform primer pockets/flash holes. Sellier & Bellot brass rates fairly high in my personal pecking order, but the flash holes/primer pockets knocks S&B out of the top echelon. As a reloader, this is not a problem for subsequent loadings, but the initial S&B factory load suffers from this problem (at least in theory).
3) Powder is difficult for us to evaluate. Obviously, some loads are cleaner than others, etc. The problem arises in that the powders used in factory handgun ammo are not of "cannisterized" quality. That is, a particular ammo lot should all have the same powder lot, but the powder lot will vary somewhat over time. For example, all Winchester USA 115 gr 9mm will not have the exact same charge weight of propellant. As the powder lot varies, so does charge weight/volumetric density.
Virtually all ammo that I have ever disassembled had a consistent charge weight within the same lot of ammunition (at most 0.1 gr variance). The notable exception was some Wolf 7.62x39 (not handgun ammo). I bought a half case of Wolf SP, mostly because Barnaul 7.62x39 SP is so darn good! I had trouble getting a recognizable group at 100m (despite the fact that my SAR-1 is pretty accurate), so I pulled a few bullets to see what was up. Assuming I was correct, the charge weights varied as much as 0.4 grains. I have subsequently tried this with other Wolf ammo, and I have never seen this type of variance (at most 0.2 gr in some 9x18 handgun ammo).
I would caution anyone from jumping to the conclusion that Russian ammo is not good ammo (at least for this reason). Obviously Tula, Novosibirsk, Barnaul, Ulyanovsk, etc. have HUGE stockpiles of powder. I cannot say that they always follow American conventions of ammo assembly. It would not shock me if the Russians used up odd lots of powder when possible. However, caveat emptor...
4) It seems obvious that bullet design SHOULD affect accuracy in handgun ammo. In practice, this is dificult to observe without extensive testing. In general, the heavy bullet weights for a particular caliber seem to suffer when an exotic bullet style is used. I would guess that bullet style/quality control is not a major factor at typical handgun distances, assuming certain minimum standards. In my reloading endeavors, I have noticed better results with Hornady, Sierra, and Nosler premium bullets. Since I tend to take exceptional care when loading expensive bullets, this may be skewed as well.
5) Some thoughts on S&B: Sellier & Bellot 9mm 115 gr FMJ may be the best bargain on the market for the CZ shooter. (I would also say this for anyone who shoots a lot of 9mm). Some S&B handgun ammo is not up to the same standard. For example, the S&B .45 230 gr FMJ load has fairly uneven flash holes. (This may not sound significant until one considers the case capcity of .45 ACP).
Are S&B and CZ in "cahoots"? Well, yes. This isn't exactly unusual in the arms business. (Winchester? Remington?) In the case of the various Czech firms, the former command/administrative system of resource allocation, production planning, etc. meant that CZ-UB, Brno, S&B, etc. were all under the same planning commisariat/management group. In other words, these enterprises were conceived as integral pieces of the overall Czech military industrial complex. Even now as independent businesses, I am certain that they tend to encourage their former ties.
Hope this helps. I would have to say that S&B, PMP, IMI, and Winchester USA are awfully good for 9mm practice. These brands are very consistent for the price. Remington UMC may be good as well, but I have stayed away from it after getting some bad .45 about a year ago. In .45, the list is pretty similar, but .45 is such a reloading proposition that I don't have much experience with cheap .45 (because it isn't cheap at all).
Although not exactly "bargain" ammo, I tend to have my best results with Speer Lawman and Gold Dot factory ammo in my CZs (both 9mm and .45). The Speer offerings (although I can't say much about Blazer) tend to follow a different price structure/marketing concept from Federal, Winchester, and Remington. In practice, I have found that paying slightly more for Speer means a dramatic increase in quality (plus the bonus of the reloadable Speer brass).