Don, good points. For our testing in 5.45 we only tested the ammo that is common and easy to get. Uly, Wolf FMJ & HP, Barnaul FMJ, and RWS HP. This is listed in the results.
For our 7.62 X 39 testing we did the same thing, although about 10 different types made in pretty much the same factories has come out in the few months since the test.
We will always be sure to give full information on the ammo tested and the rifle used as well as any other important info.
As for accuracy.......well there have been books written on this, but I will give you the short version.
Different ammo shoots to a different point of aim, even in the same rifle. In most cases the differences may not be noticeable until several hundres yards out, in others you may not notice.
Guns are machines,and no two machines work exactly the same even if identical.
Example. When I went through shrap shooters school years ago, we were issued brand new rifles from the factory. We were lined up by rifle serial number and told to shoot from the same lot of ammo. The difference in accurracy was over 2 cm. Change in ammo, the differnce was 1 cm, but different rifles shot worse than they had before. Change in ammo again......
Bing this back to 7.62 X 39 now, and factory does have a lot to do with it due to quality standards, bullet design has some effect,but bullet design really does not start having much effect until about 300y in most calibers. Many guys swear by Wolf. We found that Wolf sucked in our test. It could have been that the rifles we used all did not like Wolf.
There is no absolute in accuracy. You can have the "best" rifle
and the best ammo and poor accuracy if they combination does not like eachother. On the other hand, you can have a crappy worn out rifle with cheap ammo, and have great accuracy, if the combination works well. (I have a worn out pitted barrled 1905 Swede Mauser that shoots 1 inch groups no matter what I put through it. If I do my part)
What it comes down to, is find what shoots best in your gun for the price and go with it. I have been selling ammo for just over 10 years now. I will tell you the same thing I tell every customer who asked about accuracy.
No one can guarantee you accuracy with any ammo, only quality. Try a small amount of each brand or type, then see what works for you.