Jeff, Unique is probably the best single powder for low pressure 16 ga loading, so I would get back there and get some if it’s still there. Prices are all over the place, and there is some gouging going on (the same day last week I bought 8 lbs of Unique for $267 from one big name reloading site , another had it listed for $368.
I also have some low pressure 2 1/2” loads for Longshot, 20/28, Universal, and 572 that I trust and can send you (and have lots for other powders and components that are harder to find). PM me your email, and I’ll photograph my notes for you. And, relative to @Magnum_Bob, I know I’ve seen Herco loads in the Low Pressure Group’s updated spreadsheets.
You can use low pressure 2 3/4” loads in 2 1/2” guns using a Hartin crimp or rolled crimp to create some more working room. Many experienced people trim to 2 9/16” for the Hartin crimp applications.
You can check, of course, the actual proof rating for your old guns on the barrel flats. Different makers in different countries use different symbols (and you can google “shotgun proof marks” for a set of sites that explain the marks at various proof houses. My old British gun is only proofed for 3 tons per square inch, meaning a nominal service pressure load of 6700 psi. Keeping a safety factor in mind, I keep loads under 8000 psi, and most well under. It’s really fun to shoot light loads, and they pattern exceptionally well.
Incidentally, there is a very cool Parker owners site: parkerguns.org. Some knowledgeable people there conversing in the discussion forums.
