Shooting modern shotshells, which normally use a plastic "shot cup"/wad combo, in a rifled slug barrel would cause the shot cup to "spin" as it travels down the barrel. This would impart "centrifugal force" onto the shot and the shot cup causing it to fly outward from the centerline of the barrel as it leaves the barrel, which in turn would cause the pattern to open up considerably.... most likely making it a VERY POOR pattern to use on upland game like grouse. I believe in doing so, you would reinvent the term "scatter-gun" for your shotgun.
Considering the low cost of additional NEF barrels for your shotgun, it would be best to just send your gun it and have it fitted for the proper barrel and choke (modified) to use for grouse hunting.
:roll: :lol: :-D :wink: