Vince, et al.
I have a tremendous respect for this group and the knowledge it shares. I come with limited ballistic background, but a lot of research into shell pressures as someone trying to figure out loads to shoot from a 1937 Belgian SxS that was bored from 2.5" to 2.75"......
If you have a "duck gun" post WWII, let's go with >1950... I CANNOT say you have nothing to worry about; however, all my research suggests you'll be able shoot a quality bismuth load without damage to the gun from the shell. This does not mean an unkempt gun or wrong load, etc can't cause problems..... buyer beware....
If your gun is "pre" WWII, so, earlier than than the 1940's(?), there seems to be a gray area. This includes the years leading up to WWII. This APPEARS to be the primary transition from black powder to Nitro as a standard, additionally, it was the transition from shorter to longer shells, 2.5" and 2.75" to 2.75" and 3". This period appears questionable for the validity of the tests used, total weight of the arms, type/quality/thickness of the barrel, etc From what I can tell, best pressures for pre-WWII shotguns is in the +/- 8,000 pressure range. Better than black powder, but would be appear to be pre-"high brass" range. This is, from my limited vocabulary, the full post Damascus era. If you have Damascus, Twist, or "laminated" barrels, that's an entirely different pressure spectrum. Add tp this the actual "QUALITY OF THE ARM MANUFACTURED"..... it's whole generation of arms for the layman to confuse....
RST Shotshells sells non-tox in 2.5" for my ''37, but it's $65 a box.
So, my 1937 Forgeron has enough barrel weight to shoot higher 2.75" loads ("back bored" from 2.5" after being brought to the States) under it's Nitro Proof and inspection by a qualified gunsimth, but it appears to be a far lighter gun than those developed post-WWII, regardless of barrel strength.... it might not have enough stock weight, so, the wood dies under the firing line..... and the wood is luxurious to my hunter's 'eye'....
SAAMi standards for the US were founded in 1926; however, current ANSI ratings for SAAMI date to 1975.
I am admittedly a novice in this space. Given the risk of bad ballistics or things that didn't work our I only hope my experience influences hunters in the direction that is right for them.....
Your Belgain A5 or your dad's Model 12 are LIKELY going to shoot market Bismuth loads just fine. If your gun is pre-WWII you should investigate significantly into available loads. A 1916 Parker is a thing to behold, buy it may not be a thing to be shot....
There does seem to be a "bright line" developing in "Vintage", which appears to be pre-1916: "Classic" from 1916 to late 1930's' and "Pre-steel", which is designed for heavier loads but not a gun that can reasonably accept steel since it's from the 'lead era'.
I only hope this adds to the discussion. I am still on the journey and exploring non-tox reload so I can return this once "duck gun" back to duck hunting. The "House of Fogeron" transitioned from founder to son/step son/key man sometime around the making of my shotgun. There is a lot of history, and to not have the tool serviceable, in it's absolute beauty, is heart breaking. But to ruin it, and be a safety risk, I'm fearful.
I hope this make sense, and hope the better knowledged like Tim J have additional insight. The info I have is based on internet research, and that's generally pretty cloudy....