It doesn't take much to bring down a grouse. I typically use a 28 gauge with #7 1/2 shot; sometimes #6 or #8. A .410 is a great gun for standing or running grouse. To hit flying birds consistently, you need a lot of skill or luck. I do encourage you to stick with the .410 for stationary birds. A lot of people use .22 (I used to, many years ago) and are tempted to go for the head shot. A grouse head or neck is a small target. There are lots of good marksmen that can make that shot most of the time, depending on distance. But for all the presumed 'clean misses', there are plenty of mortally wounded grouse that fly off, seemingly healthy. Stick with the shotgun.