There is no universal metric. Yes, it is based on a set of factors/variables. I am trying to figure out what the variable are, or where ED/sd are a critical factor. kinds of like a flow chart, at x yards, for y application, a SD of z or better is needed.
There is not really an inflection point where bullets have been traveling along, then suddenly get a text message that some need to veer off course one direction and some need to veer off course another. The bullet is constantly in flight, and is constantly experiencing gravity. The bullets traveling slightly faster than others will experience slightly less time under the force of gravity, and will strike higher than those traveling faster - but compound error stats show us that it is extremely rare that the “slow shot” will also correspond to the “low shot” in the inherent cone of fire (raw group size), so there is a dilution effect, in a manner of speaking, for all compounding error functions. We know that:
1) some folks won’t care about certain degree of error contribution for their task
2) some folks won’t shoot well enough for velocity error contribution to group size at distance to be noticeable (sensitivity analysis described below)
3) there are trending spectra where raw group size initially dominates group size when gravitational contribution error, velocity variability error, isn’t yet substantially large to be noticeable (again, sensitivity analysis), but we know eventually downrange the contribution matters
4) The worse the relative variability of any given error contributor, the more likely it will be to dominate the total error. So bigger velocity variability becomes more notable in the group size earlier than lesser variability (see my target comparison photo on page 1)
So let’s run through the math here on how much velocity variability contributes to bullet strike variability - aka, vertical group size.
The Litz WEZ Analysis tool used for Cal’s articles at PRB is an illustrative tool for these compounding errors. The simulator calculates trajectory influence for a normally distributed velocity set, then applies that randomly with compounding errors like raw group size, range mis-estimation, BC variability, POA wobble, etc.
Simplified and expressed, I know 30fps ES on my PRS match cartridges corresponds to a maximum of 6” at 1000 yards in POTENTIAL vertical dispersion. In general, I also know that a 30fps ES corresponds typically to a ~7.5fps SD. High single digits, but single digits. But doing a simple RSS regression on a raw 1/2moa rifle plus .6moa velocity spread potential, that presents as an expected 7.8” group, instead of a 5” group. So the velocity variability added over 50% more vertical to my simulated group. If I double that SD to an SD of 15, which corresponds to an ES of ~60, then my maximum potential velocity contribution increases to 11”, and again, the RSS compounding error prediction increases to 12” expected group size, for a raw 1/2moa rifle, this is 7” more vertical dispersion at 1000yrds than the raw 100yrd group would predict, more than doubling the group size, and revealing the velocity as the dominating vertical error contributor in that comparison.
So let’s play with that a little in terms of sensitivity analysis and pertinence:
If I cut the rifle down to 1/4moa instead of half, the group shrinks to 11 1/4moa, only 3/4” smaller for what should have been a ~2.5” reduction. If I increase the rifle raw potential to 3/4moa instead of 1/2, the group grows to 13” instead of growing the 2.5” the raw precision increase would predict. The velocity contribution is the dominating error factor. In other words, with a 15SD, it really doesn’t matter if the load 1/2moa or 3/4moa raw 100yrd performance, my group size at 1000 will be dictated by the velocity inconsistency. Comparatively, if I have an SD of 7.5fps, contributing potential of 6” at 1000yrds, the difference between a 1/2moa raw rifle and a 3/4moa raw rifle becomes 9.5” vs. 6”, so the 5” predicted difference in 1000yrd group based on raw group size presents as 3.5” difference in reality. This means the raw precision remains to matter more in that load, and can actually influence the group size, opposed to being washed out by velocity variability in the 15SD case
And acknowledging here: this is talking about a difference between shooting 6” vs. 13” groups at 1000 yards. Alex Wheeler just shared results this weekend from Deep Creek where a guy with one of the rifles he built posted a ~4” aggregate (average of multiple groups), with a few 3.x” groups in the mix. Consistent bullet flight is EXTREMELY important for those folks. That dude can’t tolerate an extra 7” of vertical dispersion in his 3” group…
Also of note, in evaluating my load development data collected for certain rifles over the years, I have shown a correlation in the range of common charge weights that every kernel of powder contributes ~0.95fps to my velocity change. It also reveals that there are approximately 62.5 kernels of powder per grain, meaning loading precision to within one kernel, within 0.02grn which my dispenser will resolve, I am able to control my powder charge variability contribution to velocity variability to within +/-1fps. Other factors will remain to contribute to the velocity error, velocity variability, but I can at least control the powder charge variability contributor. If I were alternatively loading on a common brand powder dispenser with +/-0.1grn precision, I would have +/-6.2 kernels instead of +/-1kernel, and would contribute ~13fps potential velocity variability to my loads. Again, the same RSS compounding error sensitivity analysis can be done, showing that if my charge weight variability only contributes 2fps to my velocity variability instead of 13fps, my vertical control at 1000yrds is improved: in the trajectory sensitivity analysis above, 7.5fps SD vs. 15fps SD offered ~4” difference in group size at 1000, and this difference is showing ~11fps potential difference in ES, around 3fps SD. So simply charging on a Chargemaster vs. an AutoTrickler has potential to contribute around 2” extra vertical dispersion in a 1000yrd group. Again, for dudes shooting 4” aggregates, that 2” penalty just for choosing the wrong powder dispensing tool cannot be tolerated.
The same trajectory contribution can be calculated for other distances. At 1000, 7.5 SD predicts a max potential of .6moa increase vs. 1.1 for a 15fps SD, whereas at 600, 15fps predicts .4moa, and 7.5fps SD predicts only .2moa potential contribution - about an inch and a quarter at 600yrds. For a guy just banging 12” plates in their back 80 acres for fun, an extra 1 1/4” in their 600 yard group doesn’t matter much. For dudes shooting 2-4” groups at 600yrds for competition, yeah, that 1.25” matters a lot.
So no, there is no hard and fast “velocity SD has to be less than X to shoot Y distance,” and there is no distance which qualifies as “long range” for which velocity variability can be ignored. Only in short range do we have that luxury - OR in applications where we really don’t care about group size.
