Col. Batguano said:
I wonder if you used a cushioning plastic shot cup (like those used in shot shells) and buffered your loads with meal or something similar to what they do in "long range" premium shells in addition to the slower powder if your patterns would be even tighter?
In my other Pietta 12 SxS (the one I shot so much back in the 70's and 80's before trading off when steel shot arrived), I tried plastic wads (WAA12R) as recommended by VM Starr. He reported getting MOD patterns with the full wad, IC patterns with the petals cut to half length, and CYL patterns with the petals removed.
I tried it and had the same results. Noticed some plastic fouling, so I put a lubed fiber wad under the plastic. End of the plastic fouling and nice soft BP fowling with no swabbing, so I continued the use. I told myself there "might" be a little improvement in patterns with the fiber wad for a better bore seal, but never did enough testing to confirm. If there was a gain, it was slight, so I didn't pursue it.
One thing I did find, which sent me down that fiber wad path: With the fiber wads in place the chrono revealed a little more velocity and much more consistent velocity to boot. That convinced me that there was some gas leakage around the unprotected plastic wad, confirmed in my mind by the plastic fouling. The sides of recovered plastics right down by the base showed a little burning, for further proof. With the lubed fiber wad it disappeared.
Dunno about the meal or any other buffer. Just too much else going on while bird hunting for me to mess with it. I did find that the old Winchester Copper Plated shot patterned better than anything else, but as far as I know that's no longer available.
In my current Pietta I've dispensed with the WAA12R's and just use the fiber wads. I'm no longer trying to tighten the pattern, so it was superfluous for me and simpler without. My patterns are really clean and smooth, which is good enough.