First how many guns do you plan on making?
You KNOW you can't make just one gun, right?
You can do most of a gun with an assortment of 1/16 to 1/4 wide straights. Almost all inletting of barrel and lock.
A small deep gouge, 7 or 9 sweep, 5 mm wide. For nibbling cuts on lock nose inlet, lock internals, stabbing patchbox curves.
A fishtail gouge #3 sweep 5 and 12 mm wide. For paring inletting cuts across the grain, backgrounding any carving.
A small VEE tool for lines along the forestock, cheekpiece decoration, under line along buttstock.