I've never achieved it either. 15 yards is five healthy paces, and I can say any grouse or bird I ever shot that close, I shouldn't have.
Hunting grouse early in the season can be an excercise in frustration, hearing birds, but never getting a shot off. I usually wait until later in the year, as the birds begin to get wiser, presenting better opportunities for the dog an I. I once watched an early season grouse walk under my Gordon while he pointed another-few dogs can handle that type of situation. It also doesn't seem to happen after grouse have been shot at a few times.
I think you are on the right track with the patterning work. Keep it up, and become familiar with how your gun works at various ranges with the loads you hunt with. If a grouse doesn't give you an opportunity for a shot at the first flush, you can usually get him up again. I like to only shoot the birds the dog handles really well, as you get a better idea of what is going to happen at the flush, and how you will handle the shot. Shooting too soon is a frequent mistake, and usually results in a miss, or birdburger. I remind myself "you have time" and have knocked grouse down through heavy cover enough to know that it is true, but, it's sometimes hard to remember at the flush!
My 12 Darne is choked loose IC and loose MOD, my 16 Tobin almost the same. When, and if, you get into some snowshoe grouse hunting, you will find occasion to use a full choke gun. There are no leaves on the trees, and moving a grouse from a snow burrow, while on snowshoes, will give you lots of time for a long shot. It is incredible hunting, most rewarding, solitary, difficult, and beautiful. My snowshoe hunting gun is a long, Italian underlever hammerless single shot full choked 12, useful as a balance beam in addition to shooting iron. I haven't ever needed more then one shot at this time of year, in these conditions.
Good luck, and enjoy.
Best,
Ted
