Have the bores checked by a gunsmith knowledgeable about muzzle loaders. Most are not, today, but some are. Bore mirrors are required to look for pitting, etc., because you can't open the breech without unscrewing the breechplugs, which I wouldn't do just for inspection of the barrels. Have the minimum wall thickness determined by him or someone else with a quality wall thickness gauge. If all checks out O.K., SHOOT IT!! I hunt with original m/l doubleguns and have no more concern about their safety than I would any modern gun that had not been checked out.
If you determine it's safe to shoot, post back and several members on here can help you with load information, and where to acquire what you need.
Never hunted grouse, but from what I read and hear the guns are very similar to those needed for quail down heah'. It ought to make a fine close bird gun. It has no choke most likely, unless by some rare chance someone jug choked it.
All my best, Stan
