Its the rifle, the shooter, AND the ammo.
Shooters, like sports personalities, can have off days. In my experience, rifles generally do not have on or off days. Some ammo is inherently better in the accuracy department than other ammo (like Federal Gold Medal Match as compared with Federal green box hunting ammo).
With a pistol you can accurately determine its inherent accuracy, with a given load, by using a ransom rest. I've seen several attempts at making such a device for rifles, but no one seems to have perfected a ransom rest for a rifle so far.
No question, most people will only show you their best group of the day/month/year and call it a 1/4 MOA rifle. That is why my commentaries on my rifles (Patriot and the Hammonds), normally end with a commentary that if I do my part, the rifle certainly holds up its end of the bargain. If you look at Mel's commentary and review of the Patriot Arms Genisus, it shot lights out with most ammo and under most circumstances. Mel is a conservative guy and put in his disclaimer about he might have got a good one - and that they may not all shoot as good as his did. My Patriot shoots every bit as good as Mel's demo rifle did - meaning if I do my part, the rifle holds up its end of the bargain and will shoot 1/2 MOA or smaller groups, every group, every time out (provided I use match grade ammo).
Some rifles and rifle builders are better than others, that is why we pay some serious $$$ to buy those rifles.
For the best groups size with a non-benchrest rifle, you'll need at least a minute between shots to keep the barrel heating is a nominal amount. My best groups seem to show up waith between 2 to 2.5 minutes between shots. That is not realistic in a sniper or other competition, but you do the best you can to try to keep barrel heating to a minimum.