At the risk of picking nits........... OK.... I AM picking nits, but only in an attempt to clarify the word "aggregate". It means, more or less, "the whole enchilada" or "the whole ball of wax". It does not mean exactly the same thing as a sum nor is it an average even though in many cases it is.
To use F class as one example, let's say you shoot three 20 shot strings for a score of 194, 196, 200. On the score sheet there will be a block for "aggregate" and in this case it means the total; i.e. 590. However, in many places, the winner is determined by percentage of a perfect score; in this case 590/600=98.33%. That makes it possible to compare scores with those who shoot less than 60 rounds, a practice allowed at some matches. But if you usually score 580 at the typical F-Class match, then you could say: "My aggregate is 580 @ 600 yards most of the time, but at the last match I came in second place with an aggregate of 590."
Another example is a BR match where the winner is determined by group size. Depending on the match, the winner might be determined by the single best group of the day. Sometimes the "aggregate" score wins. In this case "aggregate" usually means average. For 3 targets, 5-shots each, you might shoot 1.70", 1.96", and 1.82". Rather than sum the group size, it is averaged to find the aggregate, in this case 1.8267".
But in both examples, the aggregate is some sort of compilation of the score for the day or perhaps the weekend. You can see that it is NOT representative of how you or you gun might shoot on a given day. If you rise to the occasion and have your best day ever, it is not fair to say you can shoot "an aggregate of 1.8267" without mentioning that it was at one particular match. In other words, aggregate can take on different meanings depending on the circumstances.
What you are asking is something a little bit different.
Measuring the precision of your rifle is complicated by the fact that there is no standard; 3 or 5 shot groups? What range? etc. For example, my cheap Howa .223 "fun gun" will shoot sub 1/2 MOA 5-shot groups nearly all the time. I'll give you 5 bucks if the next 5 rounds @ 100 yds on a windless day are worse than 1/2 MOA. However, the best group is .155 MOA. Would I guarantee that level of precision? Never.
Since I log most all my groups, I can say that out of 5352 rounds the average is exactly .723 MOA. However, that isn't fair since it includes all the bad groups from cheap factory ammo and load work up tests where you KNOW most of charge weights will produce groups less than optimum. So how well or poorly does my little Howa shoot? If I take an average of the most recent 200 rounds down the barrel (all of them 5 shot groups @ 100 yards) the average MOA is .476. Therefore, I'd say that is a fair measure.
If you first do very careful load work-up tests and produce very good hand loads, your Howa should average sub 1/2 MOA @100 yards for 5 shot groups, or perhaps better. Your 6mm will definitely do better than that. Of course the best group you ever shoot will be much smaller, but it's not wise to bet too much money on duplicating your best ever group.
