If pressures are a concern, you can send shells to Ballistic Research (not to be confused with Ballistic Products), and you will get a pressure and speed report back. Load data based on math or cyber experts can be dangerous. I got a load from a reload guru that was of by 5000psi over max for a 10a.
Ballistic Research phone number is: 815-385-0037. Ask for Tom.
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Tom Roster's services are invaluable for questionable loads
i stick with reliable published sources - you are correct - too many "experts on the web"
for low pressure short ten loads - one of the best sources is the series of "Finding Out For Myself" articles by Sherman Bell published in Double Gun Journal a few years ago. He pressure tested the loads. (he also tried to blow up on of those dangerous damascus barrels - a rusty set of Parker tubes, it took over four time the SAMI load limit to do it.)
for the original posters question about 12 gauge bismuth loads, I doubt you can do better than the BPI or Precision's book
the old guns are a lot of fun, and truth be told - many of them have probably digested loads over the years that would make a knowledgeable shooter duck for cover and have held together. I know the chamber length and minimum wall thickness on my guns -
I was on a clay course at a classic double gun event once when the shooter ahead of me touched off a load that was very loud and set him back a few paces. turned out he somehow got a 3 inch steel load mixed in and fired it in a 2 1/2 inch double. He had the gun checked out and there was no damage. Darn lucky and not recommended, the next one could send the ribs flying,
on the other hand - I hunted for many years with a 1904 Parker that was rebarreled in the 30's by Parker/Remington with modern steel 2 3/4 chambers, after shooting modern standard loads in it for about 10 years I noticed the stock was cracking at the wrist. once the new stock is on- it will be good to go for another generation.
so- whats my point? - err on the side of caution for the guns sake if not for yours, load speed and shot size kills birds, not chamber pressure