In my limited experience, you can reduce charges with fast powders by quite a bit and still get a good result. Slow powders like Blue Dot, not so much.
Something you can check is birdshot load recipes. It is safe and sane to substitute equal weight payload using your buckshot into a recipe calling for birdshot. Buckshot and slug recipes tend to be higher velocity and often higher pressure than birdshot loads.
Some manuals list several different powder charges for a given set of components along with velocities and pressures from low to high. Reloading for Shotgunners is a manual I like for that.
There are also some subsonic loads listed that may be suitable for your use. Metro Gun Systems lists these:
https://www.metrogun.com/handloads.html
and Slus R Us lists one subsonic load using Green Dot:
http://slugsrus.com/Load-Data-12GA.html
To note though is that this is for a slug load and slug loads generally produce less pressure for payload weight due to lower bore friction than shot. However, being a reduced load a quick check with birdshot loads should tell you if the powder charge is lighter than for equivalent weight birdshot loads.
Non of the loads listed in that BPI data are particularly high pressure. The highest pressure I saw was 9600 PSI and many were in the 8000 PSI range. So maybe high pressure isn't the problem. If you are loading exactly to the recipe over pressure should not be the problem. Ajay has noted that Fiocchi hulls tend to be "softer" than other hulls and produce more head expansion which could lead to sticky extraction. I have not experienced sticky extraction with Fiocchi hulls in my loading but as you've mentioned, possibly rough chambers plus more head expansion could do it?
BPI may be able to advise as well. In the past i have contacted then and they have been very helpful.
Longbow