Smallbore
After reading your post above, I can see your perspective and why you posted your thoughts in the earlier post. UK logically limits pressure standards to progressively higher pressures for progressively longer shells. Simple, logical, and fits with the progression of both shell length and gun strength. From what I've learned here from our more historically aware members, American shell length progression and pressure progession follow a similar path.

However, in my earlier post, I was referring to reloading. As such, pressure can be controlled regardless of shell length. Since I have an instrumented test gun with a chamber that matches my field guns intended to use the reloads, I have relavent data regarding any restriction that may increase pressure due to any constriction caused by a long shell/short chamber.

From your perspective, I'm sure you are still wondering WHY? Why would I want to do this even if it's possible to do safely in this manner? Well, it's probably been a long, long time since I saw a shell less than 2 3/4" in my part of the country. I could order them for something like $8-10 a box and get whatever load the mfrs chose to offer. But, being of the type that likes to go my own way often, I chose to work up my own low pressure loads, the long shell, short chamber was simply a matter of bypassing a step (cutting long shells down) if I can get the same performance.

With my limited testing, I've convinced myself that it's a practical and safe approach and eliminates a trimming step in the reloading of the only commonly available length of shells here. I've also done enough pattern tests to believe the pattern is good out of my guns with these loads.