A 12 is little advantage-if any-over a smaller gauge for Grouse if one can't come close to putting the shot in the front third of the Bird. 8 pellets into that area-and most of the time less-of the proper size to penetrate and inch or so will flop heads and they will be dead on the ground. Most of the Birds I breast out will have less than 8 shot into the front third.
The number of pellets in the smaller gauges are more than enough to do the job.
Addition:
Choke for choke the pattern doesn't get bigger with a 12 than say a 28. There will definitely be more shot inside the pattern with a 12, but a well pointed 28 will kill just as well at the ranges most Grouse are taken. Plus it will in most cases with American offerings handle better.
8 pellets sounds like the rule for turkey hunting kills to me. Never heard that for grouse. One or two pellets are all it takes to knock a grouse down.
It seemed like more pellets get through the leaves and popple brush with the 12 versus when I used my 20. Just my impression.
