Nomercy448

ES = Extreme Spread. This is the difference from your slowest measured to fastest measured shots.

SD = Standard deviation. This is an average of the difference between the average of all shots and each individual shot.

The relevance of the two: ES tells me how far spread my loads are. It does NOT tell me whether that spread is "normal" or if it was just one or two high or low values. SD tells me how far spread my loads are on average. So are they all evenly distributed along the ES? Was there ONE slow load and the rest were close together? These, among other statistical values paint a picture of the 'forgiveness' of a given load (like %RSD - which is the relative scale of the SD to the average, i.e. 50fps means a lot more to a 1900fps load than a 4300fps load).

So basically, your 50fps difference is your ES.

I hate to be the one to break it to you, but your digital powder scale is not as accurate as you wish it was. "EXACTLY" the same amount is a LONG stretch. For precision loading, you should be at least double checking your charges on a BALANCE, not a scale. Specifically, digital scales will experience instrumentation drift with repetition, and they ALL have published precision levels, often +/- 0.1grn (Hornady LnL's, RCBS Chargemasters, Lyman micro-touch...) so you might be looking at 28.3 vs. 28.5grns, and never know it because the instrument is still reading 28.3. And that's only certified when it leaves the factory, very easy to see a discrepancy of +/-0.15+ over time (had that happen with a Hornady LnL and an RCBS). Again, you HAVE to check every charge on a balance beam BALANCE - not just use a digital SCALE. Trust but verify.

Ladder testing will illustrate for you as to why you might experience high variability. For a certain range of charge weights, you'll usually see very little change in velocity even with a fairly large difference in powder charge, however, for other weights, very slight charge weight changes can cause extreme differences. Ladder testing shines a light on this.

Ladder testing is a process of load development to determine the most forgiving charge weight range. Some shooters call it the Optimum Charge Weight Envelope, or Accuracy Window, whatever. I call it forgiveness. It's a way to find a charge weight that will "forgive you" for having +/-0.05 or 0.1grn precision.

In other words, this test helps you figure out whether your selected powder will have a high ES based on a controllable variation in powder weight.

The testing goes like this:

Using one powder, one bullet, one primer, one brass type - load a few shots of each charge weight, increasing by 0.1-0.3grn per step from your starting load to a max load (same COAL). At the range - 300-500yrds, shoot all of the shots at the same POA on a tall target, noting which shots were which as you go. The long range is meant to create vertical stringing based on velocity/drop/ToF. As expected, the heavier charges run faster, so they climb up the target like rungs on a ladder. However, you'll likely find that several charge weights group much more tightly together than other weights - this is the load range you want to play in. This is the most forgiving charge weight, meaning it gives LESS variation for the same deviation in charge weight.

Stole this pic from:
6mmbr.com Ladder Testing Article - worth reading...



So based on this pic, I would expect that loads of 29.0grns to have more variation in velocity (ES) because the spread from every 0.3grns is far larger than the variation effect at say 30.0grns. Same deal for 30.8grn loads, I would expect these faster rounds to have higher variability than a 30.0grn load, based on this ladder. At 29.0 (which is REALLY 28.9 - 29.1grn), the ES is likely much higher than at 30.0. Not having any scale on the pic, lets say that spread between the 28.9 and 29.2 shots (0.3grn change) raised the POI 3". So loading 29.0grns (which is really 28.9 to 29.1grns based on +/-0.1) might mean ~2" variation, which might be that 50fps you're seeing. On the other hand, if you look at the blue shaded region, the height difference between 29.8 and 30.4grns is about half the height of 28.9 to 29.2grns. So if I load 30.0, which is really 29.9 to 30.1grns, I might only see about 1/2" variation, maybe 15fps ES.

I generally find a powder that shoots small groups for me compared to a few test powders, then I ladder test with that selected winning powder. After ladder testing, I dial in the COAL (seating depth, aka bullet jump) and neck tension to get the accuracy I want.

Clear as mud?

That's not to say that loading in the "Forgiveness Window" will produce match winning accuracy, it simply means that you're eliminating/mitigating velocity variation (and resulting vertical stringing at range) due to powder charge weight. Velocity spread can also come from inconsistent brass, bullet weights, firing too fast (ammo or rifle warming up), inconsistent neck tension, inconsistent seating depth/COAL, primer variability, etc. Ladder testing just helps to ensure you're getting a consistent combustion, yielding consistent velocities.

But, it does seem to work for me.