If lower pressures are your goal for your 2 1/2" chambered 20 ga, then I agree with using straight walled hulls and wads made for them. (Save your WAA Winchesters for your modern guns.) You also want slower burn rate powders. 20/28 is the most readily available powder. If you have it Unique or Greendot they are very good options. Cheddite, Remington STS, or Winchester 209s are your best option. If you are using once fired hulls, you can trim them down to 2 9/16" and use 2 3/4" data in them. Simply add an overshot card atop your shot column before the initial crimp process and star crimp the edges of the plastic hull to hold the overshot card in place. We refer to this as the "Hartin Crimp". I have reloads on my shelf with these crimps that are 10+ years old and they still have not opened. Its a much faster process than roll crimping and only requires you to make or buy the cutting tool. The results are not as pretty as a roll crimp, but they are very functional.
What constitutes a low pressure load in 20 ga? Good question. I would think anything under 10K PSI. Remember, the smaller the bore the harder it is to keep pressures low. We strive for service pressures under 9600 psi in the 16 bore. Since you have our data you can used it as a starting point, Especially the 7/8 and 3/4 oz loadings. You will need to drop the powder charge a tad as well. But at least it will give you a starting point.
Like you I have added a couple of 2 1/2' 20 bores to the collection recently. I am working up some loads for them as we speak. I have tested a few in my fluid barrel guns. But I haven't sent them out for pressure testing yet. I used the hartin crimp on them. One thing I have noticed about british 2 1/2" 20 bores is that the steep forcing cones do not like hulls with high brass (especially the federal hulls-- which, by the way, are one of the best and easiest reloading 20 ga. hull). So I recommend you only use low brass hulls in your reloads.
