I understand your contention about poor shots, however the grouse I lost wounded, were road shot at ranges out to 25 yards. On the ground, centered in the pattern and lost way too many runnaways. Bowl them over, up they get and run.
Maybe BC grouse are considerably tougher than your little quail.
I was told that #4's worked better. It's almost as hard to find #4's as it would be to find #2's or BB's in .410 ammo, as not likely anyone would attempt those sizes.
I had better results when I was loading my own .4120 ammo in .444 Marlin cases, with 12gr. Unique and 1/2 ounce of 6's. A .44 gas check is used as overshot wad. The patterns were super, good killing patterns to 30yards from my daughter's .410's. I had opened the chokes a bit.