Having been trained on OC and been sprayed before, I'll share some facts about SHUs, and percentages.
All OC sprays have 2 designations or numbers. One is the Scoville Heat Units or SHUs, and a percentage.
How SHUs are determined is NOT up to the manufacturer, it is actually a scientifically based equation. If a substance is deemded hot, the SHU is the ratio of water drops to one drop of the substance that it takes to completely remove all sensation of heat.
So if a substance is rated at 1 million SHU, it will take 1 million drops of water to completely remove all sensation of heat from 1 drop of that substance.
The percentage refers ONLY to the percentage of active ingredient in the can. So that a 10% can is 10% active ingredient, and 90% propellant.
Given the choice, I'd rather get hit with 15% OC rated at 500,000 SHU (quite mild by today's LEO standards), than with 5% Oc rated at 5 million SHUs.
That should clear things up a bit.
Much of this is quite incorrect, or simply misinformation for use in making valid logical decisions. The labels you mention are true, however they are entirely inaccurate and I will explain why.
OC % means almost nothing. It stand for oleoresin capsicum, which loosely means oily solution from a pepper plant. Many brands add another oleoresin like vegetable oil to increase the amount of oil. Or seperate active ingredients and then recombine them in desired strengths. 10% OC can be weaker than 5% OC. This is because the oleoresin is just the amount of pepper oil present, not the amount of capsaicin which is the pain inducing actual active ingredient. 5% OC from a very hot pepper variety or a more refined end product can be stronger than 10-15% from a more mild pepper or less refined end product. For example you would find more OC in a larger jalapeño pepper than in a small habanero pepper even though the habanero would have far more capsaicin content total. The amount of pepper oil or OC would be greater in the larger pepper.
Furthermore scoville heat units are not a scientific measurement as they are done according to taste test of judges. With modern technology there is much more effective measurements that utilize liquid chromatography and not human beings tongues to give a precise measurement. This is not to say they do not generaly give a rough estimate, but they are not a very accurate method and give quite a bit of variation for manufactures to work with (ie market as stronger according to a more favorable panel of judges).
So basicly OC% and scoville heat units (SHU) are both relatively worthless measurements. The percentage of capsaicin is really the most meaningful information you could be given, and sadly that is absent from most manufacturers.
There is however a positive side to this and that is most laws enacted were enacted by similarly misinformed people according to the information provided by manufacturers and restrict the OC% rather than the active ingredient. If you pruified the active ingredient you would be able to have next to 0% OC content and yet 100% capsaicin. This means you can often legaly create a spray as powerful or hot as you wish according to the law as long as you control the OC or basicly oil content of the spray. This is due to the misinformation you just recited as fact which is all most manufacturers provide and so has come to be taken as the effect measurement of sprays by legislators. This does not make it correct either though.
Capsaicin however is also the most expensive component of the spray as its quantity is in a finite amount in various peppers, while vegetable oil or pepper oil is cheap and in great abundance. This means manufactures are more likely to make thier spray less potent than overpowered. They also have incentive to keep some boundaries so thier business and practices are not more tightly regulated.
However to further complicate things there is something called capsaicinoids of which there is several some of which are more potent than others and of which capsaicin is only one and the most important. To further confuse people that gain a litte knowledge they list some according to capsaicinoids which is another inaccurate rating as it does not say how much of the more important vs the less important ones are contained. 5% Capsaicinoids can be less potent than 2%.
When i was certified as an instructor we had to punch a bag and cuff someone on the ground. I could fight on it but i was completely blinded and had to go by feel. The stream seems worse to me than the fog.
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This is not legal advice and you are entirely liable if you make a spray that is illegal or beyond what the human body can take and use it for defensive purposes. Something that potent will also disable you even if none gets on you because the massive content of capsaicin with the menthol/camphor will saturate the air anywhere near this liquid so heavily with fumes and capsaicin aerosol as to make you feel as if you got sprayed directly with a lesser spray.