To estimate overall ES, one must look at the round that impacted the highest from the center of the bull (whichever group that might be on), and what round impacted lowest from the center of the bull (whichever group that might be on). Add together the distances to center of the bull, to arrive at the maximum vertical spread for all shots fired, and from there the ES may be calculated with a ballistic app, roughly, as we do not know the exact velocity range of the ammo (and being off by only 30 fps for the average MV can change calculated vertical spread by up to an inch). I'm eyeballing 5-5.5" of total vertical spread, or 2.62 MOA. If the velocity range was 1030 to 1080, 50 fps ES, calculated MOA spread is 2.7, good enough for me. The ES of the Rifle Match ammo he shot was approximately 50, + or - a few fps.
Now, it is rare to witness the full ES of a large sample on a single 5 or 10 shot group. I'm not advanced enough in mathematics to try to compose a formula for how much smaller the typical ES would be, but the observed ES on the majority of 5 or 10 shot groups is indeed significantly less than the total ES for the entire sample. Say the ES on the typical group he shot was 35. Going back over my data, I would say No, this is not a good lot of SK RM ammo. I am seeing SK ammo hold sub 25 fps for 10 shots (of course, not all groups, but please see attached chart. My worst 10-shot ES for RM ammo was 35, with the others being 25, 26, 23, and 24. Likewise, Standard + only had one group go over 35 fps.
I think you've figured out what I was getting at. ES of a large sample only reveals what the "worst case" scenario "might" be. Will it ever be observed in a 5 or 10 shot group? Sometimes, but again, rarely. What shooters can gather from ES is that for a given size of target at long range, they may miss a few times due to the vertical from velocity spread exceeding the size of the target. This is not their fault, it is the nature of the ammo and inherent in the game of trying to hit a target smaller than the rifle/ammo's cone of fire. Too large of a target to accommodate the maximum ammo spread, and there is little challenge to be had trying to hit it. Too small of a target, and hitting it becomes frustrating due to the limitations of the ammo. Rhetorically, how do we select an appropriately sized target?
I am like you, I have largely left the chronograph behind in recent years. Results on target at the 50 yard distance I have been focusing on tell me all I need to know about the ammo. The groups are either tight, or they are not. Nothing to do but try as much quality ammo as possible in search of the desired results. Additionally, there is much more to the achievable precision with a certain ammo than the consistency of velocity alone (which mainly acts on the vertical plane, but can have some slight wind drift implications). Random "fliers" can occur, with the bullet winging off in random directions for seemingly inexplicable reasons. Off axis center of gravity in that bullet? Damage to one side of it? Excessive runout or canted seating to the brass? The list goes on. Good ammo is much more than simply consistent in it's velocity, it is the sum of it's whole construction.
