The answer is, yes. :)
Seriesly, everything you mentioned has an influence, including shooter skills. There are plenty more variables involved, too. The trick is to start with some arbitrarily-fixed reference points, and then figure out ways to test some variable to see how much of an influence it has on overall accuracy. In most cases, the single biggest detriment to accuracy is the shooter himself.
In my case, I'm using my Ruger 10/22 target model because it has proven itself accurate enough to overlook any remaining flaws that still might reside in it. I'm starting with Federal Champion ammo because it's relatively cheap, highly rated by other .22 shooters, and I've seen its inherent accuracy in a number of my own weapons.
Now I'm working with gauges and gadgets to see what my rifle likes best about that particular brand. Tune in tonight for more details on progress so far. And the one thing that goes without saying, that I must say, is that everything can vary from gun to gun, brand to brand, round to round. But good data can help reduce those variables to below the statistical background noise.
what he said...